Mozilla on older hardware

Help me choose my next daily driver.

I'm a long time Firefox user, but recently stumbled upon Vivaldi at work. I really like the way Vivaldi implements the side toolbar, combining bookmarks, history etc with customisable pinned tabs.

I like to keep separate work and personal browsers, and to be able to sync to Android, so I'm looking to stick within the Mozilla architecture. I'm also running fairly old tech for personal use - a 2014 MacBook Air with 4Gb ram and two cores, running Linux Mint.

So I'm looking for

- Mozilla aligned

- with a customisable side tool bar a la Vivaldi

- which runs well on old tech and linux

Bonus points if there's an android implementation, but that's not essential as I am happy with Firefox there.

I think my main options are

- Firefox, with some clever tech wizardry to implement a Vivaldi style tool bar a la https://github.com/aminought/firefox-second-sidebar (may be beyond me, not sure)

- zen (looks pretty, not sure how well it will run on my old system)

- Floorp (same comments as Zen)

- Midori (good at the moment, but due to be moving away from Mozilla I believe).

Please do let me know what you think of these options for my use case - or give me some alternative suggestions.