Modified Version ID?
You appear to be running a version of Firefox with a modified Version ID. This can be the result of having installed an extension which tries to modify your Version ID to include the extension's version, even if that extension is no longer installed. To fix this, use the following instructions:
1. In your URL bar, enter about
:config and press Enter.
2. In the filter bar, enter "useragent". The list should filter automatically.
3. Locate the entry named "general.useragent.vendorSub".
4. Right-click on it and choose "Reset" to allow Firefox to manage this value for you in the future (it will be automatically changed for you when you upgrade) -or-
5. Right-click on it and choose "Modify" and edit the version number to match your Firefox version if you wish to keep the extension's extra info (you will need to do this again any time you upgrade Firefox).
Once you've followed these instructions, you should be able to access the Mozilla Update site (assuming you are running Firefox 1.0.4 or newer).