One annoying thing when accessing SharePoint sites with Firefox is that it always requires the user to enter credentials when using NTLM authentication. It turns out, that's not a limitation, that is by design... Read this post by Patrick Cauldwell about how to make Firefox log in automatically to SharePoint sites.
Also, there's an issue in Firefox (and Safari) which disables most things which depend on popup windows, most notably, it's not possible to add web parts to a page.
Taking a look at the core.jss file, it appears that Microsoft put a fair bit of work into making this work in more than just IE, so it actually might be a bug in Firefox, not just Microsoft ignoring FF as I first thought. How about putting a vote for Bugzilla@Mozilla - Bug 463889 - Can't add web parts in Sharepoint 3 and MOSS 2007?