There's a third party version of Firefox 4 which will work with your operating system. Download from here: http://www.floodgap.com/software/tenfourfox/

Thank you, Xircal. Great info Site, indeed, but I can not see download link there..Would you direct me, please

Near the bottom of that page.

You asked for it. Now you can get it. Download TenFourFox 4.0.2.

1. TenFourFox for G3 processors
2. TenFourFox for G4 processors: 7400 | 7450 ("G4e") (Which one do I use?)
3. TenFourFox for G5 processors