In case anyone finds this from Google, it's not quite as simple as that

...
Yes, first use Add/Remove Programs in Control Panel, but if you want Firefox
completely uninstalled, you also have to delete stuff from the "Application Data" folder.
In Windows 98, go to C:\WINDOWS\Application Data\. You'll find a Mozilla folder, under that a Firefox folder, and under
that a Profiles folder. Files in the Profiles folder contain information about the themes and extensions you've installed and so forth.
Probably the simplest thing to do is to delete the entire Mozilla folder and all its contents and subfolders. I'm not sure what happens if you go into the subfolders and delete files selectively.
In Windows XP it should be similar, but I don't really know (still using 98.)
If you care, be sure to export your Firefox Bookmarks to somewhere safe like the Desktop before uninstalling
anything.