This is a guest running in Vmware btw.

do you have any sandboxing or security software on the affected system that might interfere with firefox?

No, no explicit security software. The server is hardened according to Windows Server 2012 CIS Benchmarks, but this worked with the previous version (56.*).

We do run f-secure AV, with realtime scanning enabled though. Again, there has been no change to this configuration other than an updated firefox version.

does it make a difference when you disable multiprocess-mode in firefox?

enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named browser.tabs.remote.autostart. double-click it, change its value to false and restart the browser once. (if you can't get into about:config in case it's crashing too, try to edit the prefs.js file inside the firefox profile folder accordingly).

Your More system details shows;

Crash Reports for the Last 3 Days

All Crash Reports (including 31 pending crashes in the given time range)

If any of these crashes are from the last few days,

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP- in front of the numbers/letters, click it to submit them.

The crash report is several pages of data. We need the report numbers to see the whole report.

Using your mouse, mark the most recent 7 - 10 Submitted crash reports, and copy them. Now go to the reply box and paste them in.

For more help on crash reports, see; https://support.mozilla.org/en-US/kb/firefox-crashes-asking-support

Ok.. I'm flabbergasted.. That worked brilliantly.. Can you explain a little about what we just did and is this change now made for all users on the server?

frode.svendsen said

Can you explain a little about what we just did...

firefox quantum (57 and above) is running as a multi-process application by default (that's for additional performance, stability and security). if something is hindering proper communication between those firefox processes this might lead to the symptoms you were experiencing. the pref flip i was suggesting was just a way to disable multi-process firefox and have everything run in one single firefox.exe process. on the long run this is not a good solution because this state is no longer officially supported by mozilla and might lead to other issues in upcoming versions. without having more details it's very hard to speculate what exactly is going wrong in your environment. what you should try in any case if the server is offline is to get the most recent program version of f-secure on it, as they may have made changes in the meantime accommodating the new multi-process architecture as well...

...and is this change now made for all users on the server?

no the way i've described it the pref flip will only affect this one profile/user account it was performed in. in order to set a preference for everyone that's using the server, Customize Firefox using AutoConfig would be the way to go about that.