I've been using vertical tabs (first TreeStyleTabs, now Sideberry for the last ~6 months) and I'm in the same boat.
Chrome is faster, snappier and works better on more websites I commonly use, but the fact that I cannot have "vertical tabs as trees" ruins the entire browser experience for me, so it's basically the only reason I use Firefox for the last decade or something.
Add NoScript and Firefox will be much faster than Chrome. It will make you aware of how much untrusted code poorly developed sites expect you to run on their behalf.
Well, turn off JavaScript in Chrome and you back to Chrome being faster. Turning off JS is obviously not a solution when the complaint is that (assuming the same amount of work) Chrome is faster for some JS.
NoScript doesn't turn off javascript. It allows you to selectively disable some scripts while whitelisting others. You can't use much of the modern web without JS but you can neuter the dozens of trackers and ad bloat some sites insist on running on your computer.
Running uBlock Origin in “Medium mode” [1] also does wonders (= blocking 3p-scripts and frames).
It’s interesting to see how many websites work in this mode, and the amount of crap you’re not seeing. Websites load so much faster. And, you can then (permanently, or not) easily whitelist some specific domains like content providers, etc. while browsing.
Chrome is faster, snappier and works better on more websites I commonly use, but the fact that I cannot have "vertical tabs as trees" ruins the entire browser experience for me, so it's basically the only reason I use Firefox for the last decade or something.