A number of things.
- On one of my machines it consistently refuses to open particular web sites. http://www.dbdebunk.com/index.html is one of them — IE has no problem, Opera has no problem. No error message, nothing. It just seems to pretend that I haven't asked it do do anything.
- It's a resource hog. Memory usage often tops 150Mb, and can only be reduced by stopping and restarting it. If my machine startys running slowly then I know where to look — Firefox is sitting and chewing up CPU cycles to no apparant effect.
- It's unstable. I generally get four or five crashes a week, and they're not associated with having a lot of windows or tabs open either.
It didn't used to be this bad, and it seems to be getting worse. AdBlock has been the only reason to keep using it, but that's not enough anymore. I'd actually rather have adverts on the page than put up with the all of Firefox's crap .