"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

• If you use extensions like Adblock Plus or NoScript or Flashblock that can block content then make sure that such extensions aren't blocking content.

It was AdBlockPlus

It can't be adblock plus, because I experienced the same thing and don't have it installed.

It seems like its only the first stylesheet defined that doesn't load correctly, because on my sites it was only the reset.css that didn't trigger (hence, I inherited a lot of default styles that we usually don't see).

All else looked fine.

Ctrl + R doesn't fix it, Ctrl + F5 does, however, a subsequent Ctrl + R give me the same issue once again, and I see that it tries to load the reset.css but recieves a 304 Not modified.

My first instinct was to clear brower cache, which fixed the problem, however multiple users at my workplace reported the exact same behaviour yesterday and today, and it seems to be random sites (guessing, sites that has the styles cached). Im guessing a bug in FF 3.6.10?

Ctrl+F5 bypasses the cache. Ctrl+R doesn't bypass the cache. You can use Shift+Ctrl+R to bypass the cache. So it looks that in your case it was a cache issue. That is anyway always the first to check if a web page doesn't load correctly.

In my case (FF 4.0.1), I had following symptoms and I did the following:

- yesterday I cleaned my PC (win xp) with CCleaner. It seems that after this software cleaned also the cache of Firefox and the old cookies, for some reasons, I was unable to open some websites (Facebook, but also some Wordpress-based sites). - when I used the "privacy modus" in Firefox, everything worked fine. In "normal modus" it didn't. - after reading here your posts, i did so:

• cleaned the cache
• disabled two add-ons: "Ad-block plus" and "Better Privacy".

As soon as I did this, FF started to work normally. Then, after re-activating the add-ons, it still worked normally. So I cannot understand what happened, but at least it works! :)