firefox 4 crashes very frequently, Win XP, use typical websites (facebook, google, news sites)
Here's a recent crash report: https://crash-stats.mozilla.com/report/index/6486fc5b-ab45-491f-8b02-5dcc12110609
I haven't been able to identify a specific cause but Firefox crashes a couple times per day for me -- often it gets a lot slower before it crashes entirely.
Another problem I get is occasional graphical corruption.
Finally, a lot of times if I leave my computer and come back 10 hours later, Firefox will have crashed. This happens pretty frequently.
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Also -- when I used Firefox 3.6 it was completely fine and almost never crashed unless I had well over 100 tabs open. Now it can't even handle about 20 tabs.