saved my 3k tabs session

It's highly customizable, and it works exactly as I want it to.

Firefox closed all my tabs without warning and couldn't restore them from its own session backup files, even though those files contained all my tabs. This extension helped me restore them all easily, which saved me from having to restore a large number of tabs manually, one by one.
If this extension wasn't there, I would have replaced Firefox with another browser.

It has saved me several times when I've closed windows with at least 500 tabs that I don't want to lose this makes sure i can take my time to go through them.

Also why is this not standard with firefox?

I had around 500 tabs open that failed to get restored beyond the first 10 when I shut Firefox down, which hadn't happened before. This extension brought them all back which saved me quite the headache. Of course, it couldn't bring back the groups from Tree Style Tab, but it was unrealistic to expect it to save another extension's data.

over 5k tabs recovered from a saved file. could be improved but saved me big time

For years now Firefox session restore only puts the first window at the same position, the feature to let Tab Session Manager do this is golden as windows are now always restored in the same order. The easy access to auto session backups also golden. Together with Winger extension to stash windows, finally an upgraded browse experience which works nice and safe for 1000's of tabs over a 100 windows ; ))