Waterfox[0] is another option. It's based on Firefox, integrates patches from upstream and will continue to support the old extension system.
52 ESR will still be supported until mid-June. It is supported for two release cycles after the release of 59 ESR.
Do you know how to install Tree Style Tabs for that version though? It looks like the plugin page only offer the last version.
There's a 'Version History - See all versions' link further down the page that points at the previous releases.
https://addons.mozilla.org/en-GB/firefox/addon/tree-style-ta... and look through teh list til you find something that supports FF52+
I'm a little concerned that any bugs/holes in extensions will be left open though, since it's very unlikely the extension devs are going to be backporting features/fixes.
For security reasons, yes.
For people who don't want to say bye to old extensions which are not (or cannot) be ported to 57+ there is the option to stick with 52 ESR version (Extended Support Release). According to Mozilla FAQ [1] this version gets obselete in march 8 by Firefox 59 ESR.
[1] https://www.mozilla.org/en-US/firefox/organizations/faq/