On compatibility the problems are not as big as they seem. For most Word, Excel and Powerpoint documents import and export works flawless. Only some complex documents might need a little tweaking, but for the majority you won't notice the difference. Tens of millions of users made the switch and survived!
For Firefox compatability is more a matter of the Website maintainer. As long as they stick with open standards there is no problem. But there are some obscure Websites out there that only welcome Internet Explorer users. This is only a small number of sites and many of them are cleaning up their act, due to increasing popularity of Firefox.
Let this not scare you, because Firefox is considered to be more secure and that has huge value as well. If you run into problems you can always decide to use Internet Explorer in that rare case. Personally, I tend to write to the Webmaster (email@example.com) and explain what is wrong. Firefox is gaining marketshare and thus Website owners risk losing out on business by ignoring it.
In return for the Firefox switch, you get a more mature browser with a lot of nice features. And while you're doing that, why not consider using Thunderbird for your e-mail?
This was first published in January 2005