search this forum -- for example "old version 3.6" there are several suggested links--although a reference to eng may be incorrect-- it will lead you to the right place

review resource requirements for the version you select before bothering with the download.

3.6 will likely work then there is always 2.0

http://www.mozilla.com/en-US/firefox/4.0/system-requirements/

Mac Operating Systems

• Mac OS X 10.5
• Mac OS X 10.6

Recommended Hardware

• Macintosh computer with an Intel x86 processor
• 512 MB of RAM
• 200 MB hard drive space

For older Macs that aren't supported in Firefox 4.0, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
http://www.floodgap.com/software/tenfourfox/

Or you can go with Firefox 3.6.16.
http://www.mozilla.com/en-US/firefox/all-older.html

>>> review resource requirements for the version you select before bothering with the download. 3.6 will likely work then there is always 2.0 <<<

Mea culpa. Am chagrined to admit I blithely ignored reading the system requirements before doing the download; quite uncharacteristic. Have re-learned the lesson.