What's putting you off from using firefox?
That list would certainly be way too long to fit within the scope of this thread...
But
one of the reasons is indeed the handling and behaviour wrt plugins in general and flash in particular.
Btw, for us who are happy with firefox (myself because of the NoScript, Adblock, BetterPrivacy and DownloadHelper addons):
The title of this topic may be based on a false premise: is there really such a thing as a thread's priority? The "htop" tool only deals with process priorities. Where are thread-specific priorities?
I had thought they were there by default, perhaps then is a matter of configuration.
To highlight the difference between processes and threads in htop:
press F2 -> select "Display options" -> enable "Display threads in a different color"
To toggle tree view press F5
Note: While the process numbers are shown, htop is only able to get the memory usage for all of the threads combined, and each process shows the total memory usage for all of the threads.