Unfortunately, Firefox 4 supports only Mac OS X 10.5 and above on Intel hardware. The system that you are currently using is no longer supported by Firefox (details can be found here).

You can revert back to previous version of Firefox by following these steps:

1. Open the Applications folder.
2. Drag the Firefox application to the Trash.
3. Download and install Firefox 3.6 from here.

Hi Tom,
Sorry this is a known problem, (offering a version of Firefox that will be incompatible with your OS) that should be fixed sometime, a (bug was filed).

Regarding downgrading see Install an older version of Firefox note when uninstalling do NOT use the remove settings option, that will remove all bookmarks and passwords etc.

P.S. Additional tip

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

{ I am not a Mac user and wanted to make sure this applied to Macs - info copied from TonyE answer to How can I go back from Firefox 4 to Firefox 3?]

Your instructions were very helpful, and I have firefox 3.6 back and operating. Thank you very much.

Your instructions were very helpful, and I have firefox 3.6 back and operational. Thank you very much.

Very weird day in Firefoxland

First on a Powerbook G4 running 10.4.11 I respond to an invitation to upgrade to Firefox 4 (like a lot of others). Long story short, I wind up losing the browser altogether -- including all bookmarks. Why? Because neither the Firefox homepage (or CNET downloads) makes it clear that 4.0 is for OS 10.5 only! Amazing!

Then I try to register to post a reply in a thread that crits Firefox for not making this System Requirements info upfront clear. But the Register page I’m taken to (https://support.mozilla.com/bs/users/register) is in SERBO-CROATIAN!!

Then I try to log into Chat a couple of times and when it’s not CLOSED its a long line waiting and THEN it closes.

WOW!! Are the wheels off the wagon here or what!?!

Thank you all for posting the 411 on version 4. It stopped me from replacing 3.6.16, and I am so glad I checked in with the community for the scoop!

Due to a varied history on upgrades, I NEVER replace my current FF (rename it 'Firefox36' for instance) till the new one works. That used to be tricky. but.. anyway,

Another PPC G4 user who fell for the 'download FF4!' considering I have a PPC FF4 Beta, one would think..

useragent for FF mentions PPC in the string. If there isn't a PPC version (hopefully the term is 'not yet') then warn people before they waste 68MB of Mozilla bandwidth.

Somebody screwed up royally when the switch was turned on to notify existing Firefox users of the 4.0 version being released, they didn't exclude 10.4 and PPC Mac-users from getting that notification. That mistake was rectified last week, AFAIK.

Yeah! Some note on installation requirements with any kind of hint that legacy support is sketchy would've saved me a bit of hassle today. I had to use IE to find this thread posting the link to the downgrade version. But at least I made sure not to delete my user data when I un-installed version 4. Everything went well on the downgrade install, and now I need to remember never to upgrade again.

To everyone that posted a reply to my problem;

I have been able to retrieve firefox 3.6 and am "happy" again. I thank all of you for your time and assistance.

dynamite - thanks! very helpful and concise - just what I was looking for. THANK YOU!