Is there a reason why those tabs can't be on one Browser? I think as you close the last one it saves the last one as the default that is what is happening. So unless you have a icon saved bookmark to open separate Browsers with tabs then what your doing will not work. It says the last Open Browser and tabs not previous ones. Opening and closing Browser like FF it says the last Browser/Tabs as the default when you open again. What your describing is highly doubtful that is correct.

I have three browsers open as I have two extra jobs for which I use separate email accounts and do different research. Nevertheless, I like to use them separately as it makes life easier. That way I can also have different gmail accounts and gdrives open one in each browser. Funny thing is, that it worked a while ago, but since the last update it not working anymore.

3 Browsers or 3 Browser Windows open ?

Copy/Paste session into the search box and check what maybe useful here https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/ also the word tab

@Pkshadow: Sorry, three browser windows each with at least 5 tabs open. Browsed already through many posts but couldnt find a solution. As I said all setting and configs are set. Also, it did work in the past that's why I am wondering why it does'nt work anymore...