Odd. TFF works only on OS X 10.4.11 and up - but you say your machine has 10.3.x.... That would be the problem. Did you read the release notes and TFF FAQ as noted on the TFF homepage (http://www.floodgap.com/software/tenfourfox/) ? If not, please have a look at those.
Also note that Cameron actually compiles TFF with a quad-core G5 PowerMac - so obviously every version of TFF has worked on that type of Mac...
To the original poster, I'm sorry you feel that way, but it says right on the page that 10.4.11 is required. 10.3 is not sufficient. If you got it working on 10.3 before, I am surprised that it ever worked. You may be better served by an older version of Camino or SeaMonkey.
Manfred, please try to start the browser again and give us the exact words of the error message you get (in your native language if necessary). Also: Please click the Apple menu and select About this Mac…, then give us the Processor type.