John
I'm with you all on this. Its been an annoyance for awhile now.  

I'm glad it's not just me. At first I thought maybe the Russians had hacked my computer...

I think I found it.
In Firefox, Go to OPTIONS | PRIVACY
In the History section, Check CLEAR HISTORY WHEN FIREFOX CLOSES
To the right of this same option, click the SETTINGs button.
Check ACTIVE LOGINS, CACHE and OFFLINE WEBSITE DATA. All other options, uncheck.
Restart Firefox.
 
Not sure  if the settings page needs to be exactly like mine but this is a start.
 

Same issue here on Safari, been like it for a while now.

Sorry about the many posts but the problem is related to posting so I am using this thread to test new posts.
 
One last thing (this may be THE important thing),
In the CLEAR HISTORY settings page, uncheck the OFFLINE WEBSITE DATA box.
 
Once this is set to retain website data, it seems to work in all conditions I could think of. 
 
Danny, see if Safari has an option to clear OFFLINE WEBSITE DATA when it closes and disable it so that Safari does NOT clear OFFLINE WEBSITE DATA.
 
It looks like the forum software is storing data in the browser which it uses for navigation.