I am not too sure you will get a full answer to such a question. Your OS long since stopped being supported by Apple and the Firefox versions are also very old and not supported by Mozilla * [[Firefox support has ended for Mac OS X 10.6, 10.7 and 10.8]] * (Do Not) [[Install an older version of Firefox]] Any particular reason for using old unsupported & unsafe versions of Firefox ? I usually recommend those using unsupported versions either run in a VM or have well rehearsed bare metal recovery procedures, even on Linux or a Mac there are security risks. * See also https://www.mozilla.org/security/known-vulnerabilities/firefox/ I know Fx4 was a memory hog, but would not think Fx3 should be worse than Fx2 but anyway if you have 4GB that should be no problem The system requirements for [https://www.mozilla.org/firefox/3.0/system-requirements/ Fx3] were 1/4GB & for [https://www.mozilla.org/firefox/45.0/system-requirements/ Fx45] that is only listed as 1/2GB although probably 2GB would be a more realistic figure. If you have the spare disk space have you considered installing multiple versions of Firefox, including the currently still supported Fx45 esr. Hey with 4GB of RAM you could even run Fx2 & Fx45 simultaneously.