I've gotten Firefox 3.0 to install with Kernel Ex. I'm not sure about (and haven't tried) the newer 3.x releases (like 3.6).
Also, you could try the newest version of Opera using Kernel Ex.
Per the web site, Kernel Ex is "an Open Source compatibility layer with an aim to allow running Windows 2000/XP-only applications on Microsoft Windows 98 and Microsoft Windows Millennium operating systems".
Kernel Ex is constantly being revised and updated to allow additional programs to run. The earlier versions of KEx do not work with Firefox 3.x, so to ensure the best compatibility you should always run the latest version.
The current version is Kernel Ex 4.5 RC 1 ("Release Candidate").
http://sourceforge.net/projects/kernelex/