Firefox 4 versions and later '''do not run on a PowerPC Mac'''(like yours) and require '''at least OS X 10.5''' and an '''Intel Mac'''. [http://www.mozilla.org/en-US/firefox/11.0/system-requirements/ Firefox 11.0 System Requirements] The latest '''supported version for your mac is 3.6.28''' you can download from here: http://www.mozilla.org/en-US/firefox/all-older.html [http://www.mozilla.org/en-US/firefox/3.6/system-requirements/ Firefox 3.6 System Requirements] 1. Download a new copy of the Firefox program(3.6.28) from the above link 2. Trash the current Firefox application to do a clean (re-) install. 3. Install the new version that you have downloaded(3.6.28). Your profile data is stored elsewhere in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder], so you won't lose your bookmarks or other personal data. see also: [https://support.mozilla.org/en-US/kb/Installing%20Firefox%20on%20Mac Installing Firefox on Mac] [For older Macs that aren't supported in Firefox 4+ versions, if you like, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8] http://www.floodgap.com/software/tenfourfox/ http://tenfourfox.blogspot.com/ https://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported thank you Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.