Firefox 3 is the first Firefox release for Linux that includes windowless mode (wmode) support. Unfortunately, there was a crashing issue related to wmode that a lot of people testing Flash Player 10 beta 2 have encountered, and on fairly major sites.
Great news: Thanks to the Firefox team, this issue has been solved. The fix has made its way into the Firefox nightly builds. You can download them at http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/.
The first build I know of that contains the fix is for 2008-07-08. So start with that one or later, until the next formal Firefox release. Ideally, wmode-related crashes should now be entirely Flash’s fault. But please, before reporting such crashes, be sure (using the About Firefox dialog) that you actually are using one of the very recent builds. And it wouldn’t hurt to be certain that you’re using the latest beta of the Flash Player by checking the “about:plugins” URL in the browser.