Attached an image of what I see, when following a link to a model.

Footer text is missing, model description text is missing, model image is missing, links and search box in the grey bar are missing...

Main page is the same deal - header and footer only, with the exception of a few horizontal lines, and the 2 Sketchup "branded" images at the bottom showing up.

Could you open Firefox's Web Console in that tab (press Ctrl+Shift+k) and then reload the page bypassing the cache by pressing Ctrl+Shift+r? Does the console show that the files were successfully retrieved (status 200) or any kind of error messages?

Error:

SecurityError: The operation is insecure. compiled.js:435

Tested on 2nd machine as well - error not present on working machine.

I enabled the Net tab logs, to get information closer to that in your image. I also unticked CSS reflows for brevity.

GET https://3dwarehouse.sketchup.com/model.html [HTTP/1.1 200 OK 297ms] Use of getUserData() or setUserData() is deprecated. Use WeakMap or element.dataset instead. requestNotifier.js:63 GET https://fonts.googleapis.com/css [HTTP/1.1 200 OK 172ms] GET https://3dwarehouse.sketchup.com/css/normalize.css [HTTP/1.1 200 OK 62ms] GET https://3dwarehouse.sketchup.com/js/startup_affb995.js [HTTP/1.1 200 OK 296ms] Unknown pseudo-class or pseudo-element '-webkit-search-cancel-button'. Ruleset ignored due to bad selector. normalize.css:339 GET https://3dwarehouse.sketchup.com/css/warehouse.css [HTTP/1.1 200 OK 62ms] GET https://3dwarehouse.sketchup.com/compiled.js [HTTP/1.1 200 OK 468ms] GET https://3dwarehouse.sketchup.com/js/maintenance.js [HTTP/1.1 200 OK 47ms] GET https://www.google.com/jsapi [HTTP/1.1 200 OK 203ms] Unknown pseudo-class or pseudo-element '-webkit-input-placeholder'. Ruleset ignored due to bad selector. warehouse.css:75 Unknown pseudo-class or pseudo-element '-ms-input-placeholder'. Ruleset ignored due to bad selector. warehouse.css:92 Unknown property 'user-select'. Declaration dropped. warehouse.css:117 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:260 Unknown property 'text-justify'. Declaration dropped. warehouse.css:689 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:871 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:873 Expected 'none' or URL but found 'alpha('. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:884 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:885 Error in parsing value for 'background'. Declaration dropped. warehouse.css:1026 Error in parsing value for 'background'. Declaration dropped. warehouse.css:1029 Error in parsing value for 'background'. Declaration dropped. warehouse.css:1031 Error in parsing value for 'background'. Declaration dropped. warehouse.css:1033 Expected 'none' or URL but found 'progid'. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:1037 Unknown property '-moz-background-clip'. Declaration dropped. warehouse.css:1083 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:1608 Expected color but found 'auto'. Error in parsing value for 'color'. Declaration dropped. warehouse.css:1670 Unknown property 'user-select'. Declaration dropped. warehouse.css:1697 Unknown property '-moz-opacity'. Declaration dropped. warehouse.css:1848 Expected 'none' or URL but found 'alpha('. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:1849 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:2306 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2529 Expected 'none' or URL but found 'alpha('. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:2545 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:2550 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2580 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2633 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2674 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:2677 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2688 Unknown property 'user-select'. Declaration dropped. warehouse.css:2911 Unknown property '-moz-border-radius'. Declaration dropped. warehouse.css:2964 Unknown property '-moz-box-shadow'. Declaration dropped. warehouse.css:2979 Expected 'none' or URL but found 'alpha('. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:2995 Expected 'none' or URL but found 'alpha('. Error in parsing value for 'filter'. Declaration dropped. warehouse.css:3000 Unknown property 'user-select'. Declaration dropped. warehouse.css:3035 Unknown property 'user-select'. Declaration dropped. warehouse.css:3126 Unknown property 'user-select'. Declaration dropped. warehouse.css:3207 Unknown property 'user-select'. Declaration dropped. warehouse.css:3617 SecurityError: The operation is insecure. compiled.js:435 GET https://www.googletagmanager.com/gtm.js [HTTP/1.1 200 OK 188ms] GET https://3dwarehouse.sketchup.com/img/su-logo-140.png [HTTP/1.1 200 OK 171ms] GET https://3dwarehouse.sketchup.com/img/like-sprites.png [HTTP/1.1 200 OK 437ms] GET https://3dwarehouse.sketchup.com/img/icon-downloads-white.png [HTTP/1.1 200 OK 421ms] GET https://3dwarehouse.sketchup.com/img/3dw-sprites.png [HTTP/1.1 200 OK 515ms] GET https://themes.googleusercontent.com/static/fonts/opensans/v8/DXI1ORHCpsQm3Vp6mXoaTXhCUOGz7vYGh680lGh-uXM.woff [HTTP/1.1 200 OK 156ms] GET https://themes.googleusercontent.com/static/fonts/opensans/v8/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff [HTTP/1.1 200 OK 250ms] GET https://themes.googleusercontent.com/static/fonts/opensans/v8/MTP_ySUJH_bn48VBG8sNSnhCUOGz7vYGh680lGh-uXM.woff [HTTP/1.1 200 OK 250ms] GET https://www.google-analytics.com/analytics.js [HTTP/1.1 200 OK 156ms] GET https://munchkin.marketo.net/munchkin.js [HTTP/1.1 200 OK 109ms] GET https://munchkin.marketo.net/142/munchkin.js [HTTP/1.1 200 OK 32ms] GET https://munchkin.marketo.net/jquery-latest.min.js [HTTP/1.1 200 OK 62ms]

Line 435 is hard to understand:

Wg.prototype.aa=p();function Vg(a,b,c,d,e,f,i){this.Fa=m;this.Aa=a;this.eq=c||"GET";this.vb=d;this.Bj=e||k;this.Yf=u(i)?i:1;this.Kg=0;this.fl=this.ql=m;this.Ci=b;this.Ni=f;this.Ag=k}y(Vg,dd);Vg.prototype.getUrl=q("Aa");Vg.prototype.getContent=q("vb");Vg.prototype.aa=function(){Vg.e.aa.call(this);delete this.Ci;delete this.Ni};function U(a){a=a||{};this.Bv=a.baseUrl||"/tcc/";u(a.useLocalStorage)&&(this.ua=a.useLocalStorage);this.Gd=a.localStorage||window.localStorage;this.dh=a.localUser||{id:"SYSTEM",displayName:"SYSTEM"};this.je=this.dh.id;this.ua&&(this.je=this.ki(this.dh).J())}x("warehouse.DataStore",U);U.prototype.ua=j;U.prototype.useLocalStorage=U.prototype.ua;

I'm not sure which "operation" is causing the issue.

Regarding local storage, Firefox normally allows sites to use local storage. This feature is controlled by your global cookie permission setting (i.e., accept cookies, reject cookies, accept cookies for the session only) which can be varied on a site-by-site basis in the Page Info dialog. (This is different from the offline storage permission; it doesn't have its own category on the permissions list.)

Extensions also can modify how sites use local storage...

"Extensions also can modify how sites use local storage..."

Safe mode did not work, and my test machine has the same extensions.

Okay, yes, I can replicate the problem by blocking cookies from the site.

Could you check your cookie permission setting for the site? Here's where:

right-click the padlock icon in the address bar > More Information > Permissions

Edit: Sorry, left click, not right-click

Scroll down to "Set Cookies" and look to the right side where a bullet should indicate the currently effective setting.

If it is set to Block, uncheck the "Use Default" checkbox and choose "Allow for Session". Close the Page Info dialog and reload the page.

All settings for this page were set do "Use Default" and "Allow".

No cookies were set for this site - probably after deleting, while troubleshooting.

Unchecking "Use Default" clears the issue - rechecking replicates.

I have NO EXCEPTIONS set for this site. I have no problem with leaving it set to "Allow" this way, but I would like to find the offending cookie exception - if that is the cause.

Images below indicate actions and results of the "Use Default" box, ticked and unticked, then just refreshing the page. At no point have I modified the radio button settings for "Allow" or otherwise.

The first thing that I did, when the page stopped working, was delete any cookies and exceptions for this site.

That's puzzling: if Allow is the default, why would it work on Allow but not on Default??? I may have to test in a different configuration after hours.

I wonder if having third party cookies, set to "Never", could have something to do with it...

The third party cookie setting might be relevant. But I'm not sure why changing the main cookie setting would affect that?