Did Firefox do this? Did one of the two extensions play a part? Could it be the Flash or Java plug-in (the former blocked, the latter disabled) making a monkey out of Mozilla?
It really doesn't matter, because they aren't the ones whose names appeared in the Task Manager next to some ungodly amount of RAM. Even if something else is feasting on all that system memory, it's Mozilla that gets stuck with the tab.
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Red lines show physical RAM accessible to Firefox, and green lines show physical RAM accessible only to Firefox. Upward movement matches points at which we opened new tabs.
We worked with version 1.5 on both Linux (using Xandros and the latest Ubuntu distro) and a current version of Mac OS X, in addition to Windows 2000 and Windows XP SP2 installs. In both cases, the jury is still out: Although we can't send either the Linux or Mac OS versions of Firefox 1.5 to the doghouse without better evidence, we're also not willing to let either of them off the hook just yet.
Part of the conflict is a matter of too small numbers: Of the reader E-mails we received on this subject, 10 were from desktop Linux users. Of those, only one person noted definite signs of a memory-management problem on a Linux-based Firefox 1.5 install.
Of the E-mails from Mac users, we saw only a few complaints, including one that reported freezes and the need to force the Firefox program to quit, and two that reported identical problems using scroll bars in Firefox 1.5 (a bug we can't duplicate). Once again, it's a case where maybe Firefox on OS X isn't oinking its way through acres of RAM the way it so often does on Windows--or maybe enough people just haven't complained loudly enough yet.
If Firefox is an everyday professional tool for you, you might want to hang back on installing the new version, at least until Firefox 18.104.22.168 is released. In our experience, Firefox 1.0.7 is far more stable. If you don't use the browser very often, odds are that you won't have problems, so version 1.5 is a better bet in that scenario.
We also recommend backing up your Firefox user profile before installing Firefox 1.5. Some people are reporting an issue with corrupted user profiles when they try to uninstall Firefox. If you plan to install Firefox 1.5, your first step should be to remove any and all previous Firefox installations using Add or Remove Programs in the Control Panel. (Doing this leaves behind all your customizations, bookmarks, cookies, extensions, and themes, which will be picked up by version 1.5 when you install it.)
You might also consider wiping your Firefox plug-ins, extensions, and themes before uninstalling your previous version of Firefox. It adds more work, but it's less likely that you'll encounter the problems that some people have had if you upgrade that way.