Mozilla vulnerates my Mac

I was quite happy with Mozilla until I got me a router for connecting my 
private net to the internet.

I plugged the router and started to connect the router with mozilla to 
configure the router. The result was a system crash, and my computer
Macintosh Cube
Operating system 9.1
behaved very strange. After fiddeling about for some time it seemed to 
waork again.

Now, the Mac begins struggeling again:

Visiting some Internet pages makes the computer freeze. One of them: 
Ebay, search results.

After a hard reset, starting Mozilla again, the computer freezes. 
Emptying the cache directory makes the Mozilla to be started again.

Now, what is most suspicious: I tried to delete the erased cache data 
from the trash, and the computer freezes again, without Mozilla or any 
other program running. Only when I started the computer without the 
system extensions loaded (start with sift-key pressed) the computer is 
able to erase the trash data. Is there any dependency of system 
extensions and Mozilla?

How can the dependencies be tested? The advice by Apple is to switch 
system extensions off and start the computer again. A good idea, who 
wants to do this job for me? Switch system extensions, and start and 
stop the computer - at least a one week's job of contiuous computer 
booting :-[

I run a virus program as well as erasing the PRAM. Booting the system 
from a different partition does not help either.

Is there anybody having an idea about what to do?

Wolfgang

By the way, I tried to run Opera 6 as a replacement. I receive the 
message that I need MacOS 8.6 or hhigher. Very suspicious ...