Firefox 4 requires at least OS X 10.5 and an Intel Mac. There is a third party version of Firefox 4 that runs on OS X 10.4/10.5 and PPC Macs, for details see http://www.floodgap.com/software/tenfourfox

If you prefer, you can get the latest version of Firefox 3.6 from http://www.mozilla.com/en-US/firefox/all-older.html

When you install either of the above it should use your existing bookmarks, passwords etc

me too, but on a vista machine. it trashed my Norton 360 v5; now I need to go bback to an older firefox

quincey, Symantec need to update their Firefox add-ons so that they are compatible with Firefox 4. They have indicated that for Norton 360 they plan to release an update to Norton 360 to support Firefox 4 in early May - http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=20100720113635EN&ln=en_US

Pending the update by Symantec, to use the Norton add-ons you will need to downgrade to Firefox 3.6.

To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

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

well, the first time when I hit run to install 3.6.16 it started then disappeared not to be found anywhere; seems to have worked second time.

you know, a warning that it was incompatible with current programs would have helped (such as saving me a couple of hours); as it is there is no obvious warning screen

WHY DOESN'T FIREFOX KNOW I"M NOT RUNNING 10.5 on an INTEL MAC?

Why prompt someone to upgrade if they just plain cant! Why not tell I could if I met the following system requirements!

The MacOS can do this ... Apple's updater does it all the time!

Wasting 15 minutes of anyone's time is a big deal!...Not good for FF image!

Maintaining a good image doesn't seem to be in their business model.

It's not just machine requirements. Some web sites/mechanisms don't respond well to Firefox 4, and of course you only figure that out AFTER you upgrade.

I'm going back down to Firefox 3.6 on a certain computer until the site people support Firefox 4 properly.