You should buy a new computer, or at least update to Windows XP on that one, windows 2000 was dropped from Firefox support with version 12, meaning the version of Firefox you are running is no longer secure, nor has microsoft released updates for 2000 for some time.

Firefox 13+ in Windows 2000

There's an unofficial "Service Pack 5" which allows newer versions of Firefox to be used on Windows 2000. The "problem" on Windows 2000 isn't so much that it doesn't "run", but that the installer isn't compatible with lower than WinXP SP 2.

As far as the "reminders" about updating Plugins, that's one of the downsides to using an ancient operating system which years beyond being supported - plugin updates stopped many years ago. Ignore those "reminders" or join the real world with a newer computer.

You need somehow to get updated to use supported and secure software.

Consider linux, it is free and there almost certainly will be some type suitable for your computer.

• http://en.wikipedia.org/wiki/Lightweight_Linux_distribution
• http://en.wikipedia.org/wiki/Comparison_of_Linux_distributions