Firefox seems to no longer "auto size" content in the window almost forcing the user to go to full screen to avoid scrolling

Question

You used to be able to have a window that was approx 1/4 the size of the monitor - now it needs to be almost 80% before all the content is available without having to scroll left or right

Answer 1

You have tried a lot of things, all correct, but have you repaired permissions since reinstalling Flash?
Repairing permissions is important, and should always be carried out both before and after any software installation or update.
Go to Disk Utility (this is in your Utilities Folder in your Application folder) and click on the icon of your hard disk (not the one with all the numbers).
In First Aid, click on Repair Permissions.
This only takes a minute or two in Tiger, but much longer in Leopard.
Background information here:
http://docs.info.apple.com/article.html?artnum=25751
and here:
http://docs.info.apple.com/article.html?artnum=302672
An article on troubleshooting Permissions can be found here:
http://support.apple.com/kb/HT2963
By the way, you can ignore any messages about SUID or ACL file permissions, as explained here:
http://support.apple.com/kb/TS1448?viewlocale=en_US
If you were having any serious problems with your Mac you might as well complete the exercise by repairing your hard disk as well. You cannot do this from the same start-up disk. Reboot from your install disk (holding down the C key). Once it opens, select your language, and then go to Disk Utility from the Utilities menu. Select your hard disk as before and click Repair.
Once that is complete reboot again from your usual start-up disk.
Also, how much RAM do you have? Some older G4's can struggle with today's streamed flash content.