I love Firefox, I use it all the time. Of course there's problems with it not displaying certain pages because it doesn't support some IE only things but I just use IE in that case. There are some other issues though that need to be addresses if they want to keep it up.

Ignoring security completely, there's a slew of problems that make it a pain to get everything working properly. Java for example doesn't update properly if the plugin DLL has already been copied to the plugins folder. Also when it tries to load any plugin, instead of letting it load while it downloads and does other things, it blocks and the whole thing locks up nice and solid. When you cancel downloads they don't get removed, in fact they tend to get locked by the OS and you need to hope they get freed when you restart, if not then you have to boot into command line safe mode to get rid of the 0 byte files. The fact that the older versions did not remove the previous version when you upgraded was a bit of a nuisance, I like having my machine clean, not with junk folders all over the place. Same with the uninstall, it's a slight nuisance to have 4 Firefox listings in there when you know 3 of them are garbage. And there are more that I haven't listed here.

In regard to security, the popup blocker seems to block pop unders for me quite well. In fact I haven't seen an ad in a long time. There is even an extension that lets you selectively block Firefox from downloading files from known ad servers.

Firefox is a good browser and makes browsing much easier but unless they get their act together and fix these bugs and make some of the extensions easier to use or a standard feature for those who aren't savvy to setting these things up then they won't get all those people who don't know what's going on when it comes to their computer because they won't see it as any better than IE, just more trouble to use. They may also start losing people like me because we'll get fed up that the program can't perform some simple tasks that should be fixed as soon as possible.

IE may have more security flaws but in terms of usability it doesn't generate the problems that Firefox can every now and then.