Run Firefox in Windows 95 :: Mozilla Stuff :: JohnHaller.com: "Run Firefox in Windows 95 (and Windows 98 original release)

Much of this applies to other Mozilla applications like Thunderbird as well.

Despite installer issues, Firefox actually does work well when running under Windows 95, which is good news for people still running an operating system that Microsoft no longer supports.
Installation Instructions

There are a couple tricks to getting it to work. Here are step-by-step instructions for getting things working:

1. Update your Common Controls Library using 50comupd.exe from Microsoft
2. (Win95 Only) Apply the Winsock 2 Update for Windows 95
3. Apply the Microsoft Libraries Update for Windows 95 or for Windows 98. (You will need to reboot once this is installed)
4. Now download a standard Firefox installer.
5. Run Firefox Setup 1.5.x.x.exe and install as normal.
6. Once you get to the point where Firefox launches for the first time, an error -- 'This application has performed an illegal operation' -- will occur as Firefox attempts to start its Import functionality (which does not work in Windows 95). Just click close.
7. Reboot your PC to return your operating system to a stable state
8. Now, launch Firefox again using your Start Menu shortcut and all should be well!"