Please provide more details. What web page are you seeing? How long is it there?

Since I posted, I've figured out that not all websites do this... but it did to it here for this reply page at support.mozilla.org.

It always appears that I am seeing an unformatted HTML page for the web page that is loading. I see active links and text, but sort of in an outline format and without graphics. I've seen broken pages appear like this before.

The unformatted page appears for about a second... just enough for me to recognize it... then the real page flashes up.

None of my News sites-yahoo, fox news, cnn and locals-do this. But one site that does consistently do this is www.isfdb.org. Also en.wikipedia.org.

Is this helpful??

Okay. It sounds like you are seeing the unformatted web page before the page has finished loading, and that is when the formatted page appears.

And you say this is with all sites?

I think your cache files are contaminated.

Location of the cache files; Windows: *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\ Mac: ~/Library/Caches/Firefox/Profiles/ Linux: ~/.cache/mozilla/firefox/

Close Firefox. Open your file browser to the above, and remove the folder.

No... not all sites do this. And I'm sure you're right when you say I'm seeing the unformatted web page before the actual page itself displays.

Are you asking me to delete my profile folder?? ... with all my bookmarks and settings in it??? Or do I want to look for cache files/folders within my profile folder?? I'm on a Mac.

Just follow my instructions.

or

https://addons.mozilla.org/en-US/firefox/addon/empty-cache-button/ Empty Cache Button Cache clearing made easy. One click.

Normally Firefox doesn't have a huge lag between laying out the HTML and applying style sheets to it. Perhaps there is some unusual delay in getting the style sheet in the first place?

Firefox has a tool called the Network Monitor which shows loading information in the lower part of the tab. It has a Timing tab with a breakdown of how long Firefox is waiting vs. retrieving a page. Not sure this will help resolve the mystery, but if the problem is with the connection, that might be useful to know.

I'm attaching a screenshot showing the Network Monitor for https://en.wikipedia.org/wiki/Firefox (after opening the Network Monitor, reload the page bypassing the cache using Command+Shift+r, or on Windows, Ctrl+Shift+r). Either:

• "3-bar" menu button > Developer > Network
• Tools > Web Developer > Network

https://developer.mozilla.org/docs/Tools/Network_Monitor

Or maybe I should have used this page as an example, which sounds like the problem you're having, although we do not yet know why:

https://en.wikipedia.org/wiki/Flash_of_unstyled_content

I found my problem. I went back to basics and restarted Firefox without add-ons. Everything worked. I started enabling each add-on, one at a time, until Firefox broke. My problem is AdBlock Ultimate. It has to be in compatible with Firefox v55. Same problems occurred with Adblock Plus. I need to revert back to v54 and lock it there until AdBlock is fixed. Beyond an unformatted web page flashing every time a page loaded, another problem turned up: on eBay auction pages, the description field disappeared and was replaced by a very large blank area... at least five screen scrolls worth. Again, with AdBlock disabled, everything was fine.

Can anyone tell me how to revert to Firefox v54????

Would you consider trying one other ad blocking extension before giving up on Firefox 55?

https://addons.mozilla.org/firefox/addon/ublock-origin/

For performance reasons, it's best if only one is enabled at a time.

Contact support for those add-ons and let them know.

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release https://www.mozilla.org/en-US/firefox/organizations/

https://www.mozilla.org/en-US/firefox/organizations/notes/

https://www.mozilla.org/en-US/firefox/organizations/system-requirements/

I did try both AdBlock Plus and AdBlock Ultimate, separately, one at a time. Both broke Firefox. I've successfully reverted back to Firefox v54 and turned off Auto update. I've re-enabled AdBlock Ultimate without problems. Both the eBay problem and the flashing unformatted page problem are gone. I've already sent a message at the AdBlock Ultimate site... no reply yet. I guess I need to send one to the AdBlock Plus site too.

MacEvansCB said

I did try both AdBlock Plus and AdBlock Ultimate, separately, one at a time.

Yes, I did read that, and I suggested a different one.