hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

Does this happen sometimes / alot?

Is the address something you typed in or a link or bookmark?

The Problem was with Norton's toolbar which I had used before; Sure will miss the convenience that it provided

Yes, many have reported Norton. Good luck and happy computing.