Switched to Sidebery from Tree Tabs, definitely better, especially regarding performance. I have 300+ tabs, since I switched to Sidebery, Firefox's RAM usage is half.
https://support.mozilla.org/en-US/questions/1396277
Sidebery solved that successfully.

The only option I miss is unloading tabs/pages upon a predefined time interval. I'd set it to 1 week.