Firefox 4 will not work on OS X 10.4, it requires at least OS X 10.5 and an Intel Mac.

There is a third-party build that works on OS X 10.4 and PPC Macs, for details see http://www.floodgap.com/software/tenfourfox

Thank you! Like the site says, "from our years of using Macs, (it's that) they outlast anything else out there... These are our computers, dang it. We paid good money for them. They still work. "

I had the same problem. I'm running 10.5 but on a Motorola chip. The Firefox download page is not clear on OS and machine requirements. To fix the problem, trash the newly installed Firefox 4. Download Firefox 3.6.16 and reinstall. All of your preferences and bookmarks will still be there as they are contained in a different place. Hope this helps.

thanks for letting us OS X mac book pro users know before we went to all the trouble and frustration. no where does it say this before you are "strongly recommended" to down love 4.0 i had to reinstall 3.6.13 or whatever and i cant get rid of the dmg from my desktop and the disk image keeps popping up. wtf is going on. i am not the most computer savy but i did everything right. i just want my old life back. please help.

Where can I get a copy of 3.6? I no longer have it on my drive. I tried to install the 4.0 and got the slash symbol too. Macs do last FOREVER! I am using 10.4.11. Man, am I pissed. I use this Mac for graphics full-time from home. It is Furniture Market time and I am slammed with work. Not a good time for this to happen. When I try to get the 3.6 from the Firefox site, it list 4.0 under details. Help! I was loving me some Firefox until now! Let me know if you know where I can I get a copy of 3.6. Thanks.

I am writing this on Safari because I am downloading and reinstalling 3.6.16 - which was not easy to find. I downloaded 4.0, had the same problems as the this thread is talking about and now I can't find 3.6.16 on my machine anymore because I deleted it to make way for 4.0. I really hope I'll find all my bookmarks and tags,. Not a cool way to start 4.0. I have OS X 10.5.8 but no Intel inside! Where was that warning before I downloaded?

Good, at least all my bookmarks, tags, and settings are still there. Still an unnecessary headache that could have been avoided with system requirements listed before download.

same thing happened to me just google search mozilla firefox 3.6 and go to the page and download the language you need and reinstall it. it will put your old version back on but be forwarned that you will then have the problems i do which is i cant get rid of the .dmg on my desktop and the disk image in my finde. if i trash it, firefox doesnt work anymore and i had to reinstall it YET AGAIN bullcrap

Where do I find a useable 3.6 version? I googled and it said 3.6 but reverted to downloading 4.o again! I too have lost use of Firefox with trying to download this version that is not supported by my system... Mac 10.4.11

Is there any solution? Is there a link for the older version?

Using Mac 10.4.11 When I launched Firefox today, I got a message urging me to upgrade to 4.0. Now, I NEVER get these software upgrade notices unless they're compatible with my system so like a fool, I didn't bother to scrutinize the OS requirements. And nowhere in the installation process was I warned that it wasn't compatible with 10.4. I just followed the instructions, which unfortunately ended up deleting my current 3.6 version and like everyone else here, when I tried to launch it only THEN I was told it wasn't compatible.

when it was too late to recover 3.6. What genius devised this evil plot?

So now I'm also trying to simply restore the 3.6 version which is no where to be found. I guess I will just give up on Firefox if they're this user-unfriendly and stick with Safari.

Have the same prob too. What a mess!!! Firefox 4 works with Windows XP, but not with Mac OS X 10.4. And the security updates for Firefox 3.6. will be only maintained for a short amount of time. http://www.mozilla.com/en-US/firefox/all-older.html Sorry guys, but this is a FAIL!

Just another Mac user that got bent and spanked. Downloaded the 4.0 upgrade and lost it all. This is ridiculous.