I've been using Firefox almost exclusively as my web browser of choice for more than 6 months now. I really like it. I've only experienced one major glitch so far, a corruption introduced when I tried to restart the browser after installing what turned out to be a tab management extension incompatible with the current version. I had to mess around with the stuff in the "chrome" folder and that's no fun.
Otherwise, it's been smooth sailing. The number of web sites that require Internet Explorer seems to have been gradually diminishing, so I have had little call to use that woefully out-of-date tool. I can do my travel reservations without having to use IE, for example, and that's a real plus.
I did read some commentary on /. that the days of Firefox's avoidance of most of the security problems that have plagued IE may be numbered, given that there may be issues connected with the lack of security surrounding some of the extension download processes and sites associated with Firefox. That's too bad, but I figure it was inevitable. The bad guys seem to be everywhere.
If you haven't tried Firefox yet, I strongly suggest you do, especially if you work constantly with Internet based sources.