Hi meok, Firefox 4 versions and later do not run on a PPC Mac and require '''at least OS X 10.5''' and an Intel Mac. see [https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-10-4-or-powerpc Firefox no longer works with Mac OS X 10.4 or PowerPC processors] The latest '''but unsupported''' version for your os is 3.6.28 , you can download it from: http://www.mozilla.org/en-US/firefox/all-older.html 1. Download a 3.6.28 copy of the Firefox 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 probably you won't lose your bookmarks or other personal data, if you don't already totally uninstall firefox(user data and settings like bookmarks, extensions, etc. located here: ~Library/Application Support/Firefox so you must uninstall it separately, to totally uninstall firefox). see also : [https://support.mozilla.org/en-US/kb/install-firefox-mac?redirectlocale=en-US&redirectslug=Installing+Firefox+on+Mac Installing Firefox on Mac] thank you Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.