Forward, not really. Mozilla has very much already used the opportunity to rip out tons of legacy code and so most legacy extensions are already not anymore compatible, even if you tell Firefox to load them.

And just as well, most of these performance improvements would not have been realistically doable, if Mozilla were still forced to not break compatibility.

What you can do, is to move to Firefox 52 ESR. It won't have the performance, but it has the legacy extensions and will continue to get security updates until June 26, 2018.

There's also some forks, namely Pale Moon and Waterfox, which seem motivated to try to keep this legacy extension system alive for longer, so maybe that works for you, too. Personally, I doubt they're able to. The old extension system was a major maintenance burden for Mozilla, so I can't imagine the far smaller teams behind Pale Moon and Waterfox taking that on.

Lastly it should be noted that Tab-Tree-like extensions will be possible with the new extension API and some implementations are already underway:

https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/

(Not a tree, but tabs in a sidebar: https://addons.mozilla.org/en-US/firefox/addon/tab-center-re...)

I don't know about Tab Tree, but the popular Tree Style Tabs extension is now a WebExtension that is compatible with Firefox 57+.

https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webext...

Tree Style Tabs is a webextension now.