Firefox version from 3 onwards do not work on Windows 98. Older versions of Firefox do, but they are not supported any more and so will have unfixed security problems.
Windows 98 itself is no longer supported by Microsoft and has a number of serious security problems. Taking a Windows 98 computer on to the Internet is very dangerous - your computer is likely to be taken over and used for sending spam or worse.
In most cases, if your computer works fine for you, no-one should be able to force you to upgrade. But when you take that computer on to the internet, more people are affected than just you. Allowing your computer to contribute to some of the internet's problems like spam is not a polite thing to do (and it'll make your computer slow and crashy too). You need to either upgrade to Windows XP (which is still supported for a little while longer) or get a new computer.