It works pretty good for my use case. Just read the longer 5 star review in this thread and follow their direction, and it should work for you too.

Thanks for improving our Office365 experience. ;-)

doesnt work! Installed it on W7 PC & tried it on my banks webpage which always annoyingly times out but it still does this!

The verbiage of the 4 settings isn't super-intuitive, but considering that this is the only session keeper app I tried across 4 different web browsers that actually worked, that's totally forgivable.
-- Make sure that after you create a Session rule set, you either refresh your target URL or navigate away and back to it, then the session keeper will start working
-- If you pretty much never want to stop it while your browser is open, pick some garbage news link that you'll never visit again as the Stop session URL
--the URL to Ping setting will probably be the same URL as the Start Session URL two steps above
--How often to ping? I put the same number of minutes in both fields. Not sure if it's the best thing to do, but it seems to work.
--I suggest that you check Show Notifications the first couple times you try to use this and set the "How often to ping.." minutes to 2 minutes, just so you can get some fairly quick reassurance over whether or not it's working.
-- Remember Changes -- that's the same thing as a Save button. If you look in the Saved Sessions drop down menu at the top of the page, you'll see what you just saved. that's when you want to refresh or revisit your target URL.
--Best of Luck!

Simple & Easy in using. A great Addon from the programmers!! Just installing, configuring and the session never end. Just need to click on OPTIONS and than filling out the forms, back to the website and making an reload of it - and the addon starts to work !

Saves me quite some time and annoyance, thanks!

I have no idea about this "URL" thing. What I was hoping for was the old "SESSION MANAGER" (Before ver 60 obsoleted it) to SAVE MY "TABS/BOOKMARKS..." when Windows requires a reboot, system crashes, have to go to work... or to just SAVE my tabs/sessions and reload when you come back and log on.

And to reload all my tabs when was able to log back on. It is hard to belief someone has not created a replacement for "SESSION MANAGER" To simply save your tabs if you have to reboot, logoff your PC, etc. This should be a STD FEATURE OF FIREFOX.