Some people have waited for Firefox 3.5 for some time and when the final version was released few days ago, some people were disappointed due to many bugs that should not be there for the final version.
That's why Mozilla team decided to release 3.5.1 as soon as possible (which is supposed to be released by this month, but no date has been confirmed yet). I found it very stable, unless for one problem that only occurred on my laptop which is under Linux where it couldn't do anything as no output comes from firefox invocation. This is the first problem i encountered with Firefox since i have used it before 1.0 has come out.
Type Firefox 3.5 Bugs on Google and you will get lots of information saying that Firefox 3.5 is too premature to be released due to lots of regression and unfinished features.