Memory issues? I have have never had any memory issues with Firefox and it's been almost crash free now for the last six releases.
It's 2016 now so maybe it's time to upgrade your box and it's 4 GB of RAM.
EDIT: How is Firefox eating up to 3Gb of RAM on your machine? I never seen mine go over 1.5GB and that's been after a week of not closing it (64-bit version/no Flash installed).
I've easily gotten it up to 4GB. Five windows, several tens of tabs per window. Webapps like GMail (2 instances of it), GCal (also 2), Facebook, Tweetdeck, Slack, etc. open all the time eat a ton of RAM, and just various complex websites/apps sitting open over time will often leak a bit.
I just restarted FF an hour or so ago, and haven't even had it reload most of the tabs that used to be open (FF restored them, they're just not loaded yet as I haven't tabbed to them), and I'm already at 1.5GB resident with probably 15 active/loaded tabs.
It doesn't sound like you are doing anything much different than me except I don't use Facebook. I normally keep my browser open for weeks at a time and never break 2GB unless hit some crap website. Addons?
200 tabs split in 12 windows, each window seem to carry a big overhead. Btw, I said "2 to 5 GB of free RAM", so that means I have more than 4 GB total... pay a bit more attention before being pedantic.
I read your comments just fine but you weren't making sense but now you explained it. 200 tabs in 12 windows? Yeah, nobody cares your browser is crashing now. You can switch to Chrome and maybe it won't crash as much but it will use more memory.
Personally, I would reevaluate how you use browser.
