Firefox and Chrome are crap browsers. I used to be a long time FF supporter and prior to that I was using Netscape, Phoenix and the Mozilla suite. Just by the fact Firefox chooses to use Webextensions for add-on code is ridiculous. That type of code is plagued with security issues. All FF does now is try hard to copy Chrome. My browser of choice now is Pale Moon which uses the old tried and true add-on code XUL. However, many websites especially Google's asinine Recaptcha play hell with it due to useragent sniffing. Although with Recaptcha there's more involved on what it's sniffing in your browser. Try to pass a Recaptcha in FF or Chrome and you can do it in one to two tries. Try it in Pale Moon and it's like ten tries if you can pass it at all. I call that blatant browser discrimination.
For anti-virus I don't use one. I used to use Bitdefender Free, but I just have that on my parent's computer. I mostly depend on Sandboxie for my browser. I used to swear by the add-on NoScript saying it was a gate before your anti-virus, but now a days websites are loaded with JS and everything else and I grew very tired of having to whitelist and/or temp allow scripts which defeated NoScript's whole purpose. SO Sandboxie helps contain all the possible bad in a sand boxed environment never touching my computer. I do use uBlock Origin in PM with several lists including the malware domains list which helps. Plus, the blocking of Ads helps keep malware laced Ads at bay. I scan ALL
downloads at Virus Total, and once in a while I'll scan the entire computer with many different things like HerdProtect portable, SuperAntiSpyware, TdSSKiller, Sanity Check, Stream Armor, Ring3 Hook scanner and many other things. Nothing ever found and I don't even use Winblows updates that seem to lack QC and can cause more trouble than they are worth. I see it time and time again on all the computer forums I'm a member of. I maybe have four updates and those pertain to my motherboard drivers and one for Sandboxie to work.
Next time you have a possible browser issue there are many things you can try. As you already found out, disabling add-ons is a starter, but I'd just use safe mode next time instead. To invoke safe mode in your browser, at least for FF and Pale Moon. Press and hold the shift key and launch FF.
Another thing you can try is portable FF as a plain vanilla test bed. Portable Firefox can be had at the portable Apps website.
Something else that could go arye is the
file. This file is the file that holds your browser settings. Ever go to about:config and change things there? That's where it's all stored. If you temp remove it a new one will build upon the launch of your browser for a clean slate. You can open the prefs.js file with Notepad++ and look in there and see what pref might just be the culprit. You can even compare one prefs.js file with another using Winmerge or other file comparison program. The location of the prefs.js file is in your profile folder. For Pale Moon it's in:
C:\Users\[user name]\AppData\Roaming\Moonchild Productions\Pale Moon\Profiles\[profile number].default
A similar path is used for Firefox in AppData. You may have more than one profile folder. Usually the one with the most stuff in it is the one you're looking for. Another way of finding the prefs.js file and other files in practically a few seconds is by using Everything.exe by VoidTools. This is a search program to find things on your computer which works a lot better than the Windows search. I use it all the time and turn off indexing which helps with the write cycles on my SSDs. And you can find text withen files using FileSeek. It will even browse though DLLs so long as your search query matches what's in the DLL. To edit DLLs use ResourceHacker.
Okay, more Info. then needed.
I just checked both of my phpBB driven forums with portable FF version 67.0.4 and there is a split second of non-CSS like behavior. I'll try the latest version of FF latter and see what happens. Using portable Chrome version 74 and it renders the index fine for both of my sites.
Now excuse me while I squeeze a squid for ink.