Microsoft no longer provides updates for W98SE.
Per http://v4.windowsupdate.microsoft.com/en/default.aspMy advise is to switch to another browser.Important: End of Support for Windows 98 and Windows ME
Effective July 11, 2006, support for Windows 98, Windows 98 Second Edition and Windows ME (and their related components) will end. Updates for Windows 98 and Windows ME will be limited to those updates that currently appear on the Windows Update website. Microsoft will no longer create new updates for Windows 98 or Windows ME after this date. This notice reflects the policies of the Microsoft Support Lifecycle. For more information, please read the Support Lifecycle policy. To enhance the security of your computer and to continue receiving updates, we recommend upgrading to one of the currently supported versions of Windows. For more information, visit the Microsoft Windows website.
I'm using Firefox 2.0.0.19 (updated yesterday), on my old machine.
Unfortunately the Firefox 3 series isn't compatible with Win 98/SE, and yesterday's update may be the last one for the FF 2 series.
I may be looking for another browser myself.