Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Do a clean reinstall of the current Firefox release and delete the Firefox program folder before installing a fresh copy of the current Firefox release.

• Download the Firefox installer and save the file to the desktop
  *https://www.mozilla.org/en-US/firefox/all/

If possible uninstall your current Firefox version to cleanup the Windows Registry and settings in security software.

• Do NOT remove "personal data" when you uninstall your current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including data in profiles created by other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you uninstall and reinstall or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.

If you keep having problems then create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Hi cor-el,

Okay, I've spent the last few hours doing as complete an uninstall of Firefox as I am able to. I'm okay with losing the logins (I did that long ago when trying to fix the previous problem) and I backed up my bookmarks.

So I uninstalled, deleted the Mozilla folders and cookies, rebooted my computer, reinstalled from a fresh download, and even disabled my AV's Internet features. And........ It's still doing it.

Before doing the reinstall, I tried rebooting my computer in safe mode with networking and curiously it seemed fine - the styles were as they should be... at least some of them came back. I rebooted in regular mode and tried FF and at first it seemed fine again - the styles were back in Twitter.com and in the Add-on search page. But the thumbnails were missing in my list, so not everything was behaving normally.

I have NO idea why it would be doing this. I've done a full scan using ESET antivirus -- nothing. I scanned with Malwarebytes and something did come up, but it's quarantined now and I don't think it would have been part of this anyway.

Did I miss something you suggested that I could try? What else can I do?

Thanks for the help.

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

You can check the log in the Web Console for error messages and the Network Monitor (Firefox/Tools > Web Developer) to see if content fails to load or is possibly blocked.

• https://developer.mozilla.org/Tools/Web_Console
• https://developer.mozilla.org/Tools/Network_Monitor

Use Ctrl+F5 or Ctrl+Shift+R to reload the page and bypass the cache to generate a fresh log.

Hi cor-el,

You were right! After days of testing different possibilities, I finally determined what the problem was. I'm using ESET antivirus, and it has a feature called HTTPS Scanner. Turns out it is apparently on steroids because it will block styling of all HTTPS pages, as well as Firefox's own add-ons pages. When I disable only that feature, it all displays correctly.

It's very curious, since this was NOT happening back in February when I stopped using ESET and started using Trend, but it began when I reinstalled it last week and removed Trend. I changed back to ESET because Trend had been causing yet another problem: it would reset the security and privacy settings! And the Trend problem only started after Firefox updated to v.45.

Yet, in both cases, everything worked fine in Chrome -- no settings were mysteriously reset and everything displayed fine.

So that's 2 for 2 antivirus programs that used to work fine with Firefox, but recently caused problems in Firefox only. It suggests that maybe something has been changed in Firefox recently (esp. starting in v.45) that makes it trigger an overreaction from these antiviruses. A search through this and another Mozilla forum show that I'm not the only one having problems this way.

Perhaps the Firefox developers need to look into this a bit further to see why it is triggering these overreactions?

In any case, thank you very much for your help. Much appreciated! :-)

The problem with TrendMicro resetting privacy setting is likely a problem with the TrendMicro Password Manager component.

Yes, it was the password manager, but I'm not so sure that this was simply a Trend problem. I had a different problem with Firefox after uninstalling Trend and reinstalling another antivirus (ESET). It would display certain sites only in plain HTML text (no styling or layout). Turned out to be part of ESET's web protection that was being overenthusiastic. That's 2 out of 2 major antivirus programs that didn't have a problem before with Firefox. The problem with Trend started when Firefox updated to v.45. I don't know when the problem with ESET started, but it wasn't happening when I uninstalled it last February. I'm not convinced Firefox isn't involved in what's happening somehow.