Start Firefox in Safe Mode to check if one of the extensions (Firefox/ Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance). To Enable SafeMode You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut. Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running. Once you get the pop-up, just select "'Start in Safe Mode" If it works in Safe Mode and in normal mode with all extensions (Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears. Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: "Firefox > Quit Firefox"; Linux: "Firefox/File > Quit")

u can also check for problems with current Flash plugin versions and try these: disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later) disable hardware acceleration in the Flash plugin

Flash is up to date, no real player installed.

Regarding safe mode. Its not feasible, as I have not been able to reproduce the problem. I can go a couple of weeks without it occurring and I cant really run FF without Extensions for that amount of time.

If YouTube is slow, stuck or weird, and people suggest that you either change or install stuff....DON'T TOUCH A THING. Please don’t mess with java scripts, firewall settings, flash players, anti-virus utilities, activeX controls, popup blockers or different browsers. And definitely avoid any software that some people on this page are trying to get you to buy. None of those things will help you, because your computer and YouTube were working perfectly fine at one point, right? The first rule of thumb and best option should ALWAYS be: don't disable, erase, uninstall, switch, download, alter, add, change or install anything. That'll only make matters worse. Every day, YouTube often shuts down many of their servers to update stuff. Please be patient if YouTube is slow, stuck or mangled. Transmissions from YouTube to your computer are actually done in small chunks of "Flash" at a time. That is why sometimes you may only get bits and pieces of text, video, the spinning circle and/or a blank screen. Your issue may fix itself within a few hours or by the next day. If your YouTube continues to behave slow, stuck or strange, it's very possible that your directories of "temporary internet files" and memory (often times called "cache") are full to the maximum and just can't take anymore, the end result being that YouTube videos either A) start/stop/start/ stop, B) grind to a halt, or C) never even make it onto your screen. "Temporary internet files" are tiny bits of webpages that get permanently put on your computer. They will remain forever and ever, gradually plugging up your computer (just like hair slowly clogs a sink drain) until you get rid of them, or "clear your cache" including "cookies" and "history". If you're not sure how to clear your "cache", or if you would like extra 'tips' on making YouTube run even better (without having to buy or install anything), click my avatar and send send me a message -- ideally with your YouTube or regular email address. I will guide you step-by-step through "cache" clearing and my 'tips' in plain English with full explanations. Note 1: if you have Google Web Accelerator (or something similar to it), dump it, burn it, trash it, disable it, uninstall it; -- do all that you can to get as far away from it as possible, even though this contradicts all my advice. Not only does it conflict with YouTube, simply clicking on "Don't Accelerate This Website" will not help you whatsoever. Note 2: longer videos only require "Flash" and "Shockwave" in order to play smooth. Other than that, avoid all the sales pitches that some Yahooers are shilling, because they are really disguised salespeople and peddlers. Their "answers" have nothing to do with your question, and their wares may actually worsen your problem. Besides, what kind of stupid answer is:....[my paraphrase] "Although you can't watch or access videos properly, get this downloader." Note 3: an unusual fix that once worked for me was altering the URL "www . youtube . com" to simply "youtube . com", (or you can even try "ca . youtube . com").

I solved the problem by setting up another user account on my computer. I will slowly and grudgingly migrate everything because I have tried EVERYTHING. I am an intermediate tech support person, so trust me I did everything imaginable first.

Control Panel > User Accounts ... > Add or Remove User Accounts > Manage Another Account > Create a New Account

Then log in on that account and see if you still have the problem. It was fixed immediately for me, which tells me it was stuck in some deep setting or registry entry.