I'm guessing media players (or other plugins) are involved. Based on your reference to XWindows, I'm guessing you run a flavor of Linux?

In case extensions or custom settings are involved, could you test using a new profile? Close Firefox and start up in the Profile Manager as described in Profile Manager - Create, remove or switch Firefox profiles. Any time you want to switch profiles, close Firefox and return to this dialog.

Any difference with problem sites?

Firefox 19.0 Mozilla Firefox for Ubuntu canonical - 1.0 Lubuntu (Ubuntu 12.04.2 LTS) with Linux-3.2.0-38-generic (i686) hp pavilion ze4800 with mobile AMD Athlon XP2800+ and 1 GB RAM

(That's what I'm running today, and yesterday the software was probably the same before it got destroyed. The lack of system information in my question is because of bad design on the support.mozilla.org page where I submitted the question. When I clicked on the question, it opened a text box for editing the question, so I hit enter, wanting to close the box, but it sent the report instead, before I could add any system information.)

The crashes have happened with all plugins disabled. (I thought that would go without saying, because the crashes also happen in safe mode, which is supposed to disable all plugins.)

I'm using Chromium for web searches and for communicating here. (I think that it shouldn't be assumed that people reporting severe Firefox problems are using it to make their report. Yet this support.mozilla.org site was trying to give me a download for Firefox to use while I was writing the report. That's really weird.)

I don't know whether Safe Mode disables all plugins. In the new profile test, you will need to disable the nonessential ones manually using:

Tools > Add-ons > Plugins category

The crash that causes logout is repeatable on epitonic.com with all plugins disabled, and "allow smooth scrolling" unchecked and "hardware acceleration" unchecked, in safe mode, and allowing pages to finish loading before attempting scrolling or any other actions. (Without playing any music of course, because plugins are disabled.)

Chromium doesn't crash on epitonic.com. (Although it does have audio hangups and scroll hangups for a few seconds at a time when navigating the site, suggesting that the site is a really hard test case for browsers.)

Does disabling JavaScript has any effect?

You can check for problems caused by recent Flash updates and try these:

• disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed
• disable protected mode in Flash 11.3 and later
• disable hardware acceleration in the Flash plugin

• http://kb.mozillazine.org/Flash#Troubleshooting

In Firefox Safe mode these changes are effective:

• all extensions are disabled
• the default theme is used (no persona)
• userChrome.css and userContent.css are ignored
• the default toolbar layout is used (localstore-safe.rdf)
• the Javascript JIT compiler is disabled
• hardware acceleration is disabled

• plugins are not affected
• preferences are not affected

I'm going to consider this problem "worked around" by just avoiding epitonic.com when using Firefox, because I can't find any other page that causes it, after trying about twenty sites that have music and videos or lots of graphics. (I thought there was one, but maybe it was only when I had epitonic open in another tab. Tumblr is very slow navigating though.)

After turning off JavaScript, Firefox still crashed on epitonic.

Epitonic used to say "epitonic loves Firefox" back around 2006, but now I think it should be treated as a Chrome demo site, only to be opened with Chrome.

(By the way, I use Adobe flash version 11.1 because only versions up to 11.2 are available for Linux, and 11.2 crashes on my hardware, but 11.1 works with everything I've tested.)

The Epitonic site seems fine on Firefox 19 on Windows 7. The sessions page is a bit slow to load all the images, but when clicking through to a session it's strikingly fast. They've just done a great job keeping those pages light, it appears. And the MP3s download pretty quickly from Amazon S3. Impressive design. (I'm spoiled by a fast connection here at the office; I'm sure more patience would be required on DSL.)

Note: I use NoScript and did not allow the Facebook.net connectivity while using the site. Not sure whether that is relevant.

I'm paying for high-speed cable Internet at home. What I end up getting sometimes looks more like dial-up, because of software-emulated graphics and slow-downs on scripts or HTML5, whether I'm using Windows XP or Lubuntu. So I should get a new computer with a graphics card.

The problem is not solved. I still get crashes in Firefox when loading graphics heavy pages, and they don't generate crash reports. Sometimes the crash stops the desktop, so all applications are closed and I have to log back in, and less often the crash goes into a black screen where I have to power down to restart the computer. The crashes happen even if the page just has a large picture or a lot of GIFs, not anything to do with Flash. The crashes never happen using Chromium, even on the same pages.

I can't just wander around the Internet using Firefox, for instance following links from Reddit or Metafilter or Google, without risking damaging the hard drive so much I would have to install Linux again. I've been using Chromium a lot more because of that.

I'm guessing the bug is a reversion of some problem with scrolling or high memory use with old graphics (since I'm running Radeon IGP 320M on chip and mostly software graphics) that was solved in Linux but Firefox has reverted on. Maybe Firefox is now calling some graphics library function that it's not supposed to call on systems that don't have the latest graphics features, or maybe the fixes in Firefox for the mouse scolling bugs that used to make it lock up on a lot of systems a few years ago are not as fixed as they think they are, or maybe Firefox is calling some sort of memory management for large graphics loads that is inappropriate for systems that don't have the latest graphics features. (I never have problems with lack of RAM with any other program, with this laptop having its maximum of 1 GB, or when it was running at only 512 MB, except it was slower then.)