Regardless how recommended a given extension is, all tab manager extensions require access to all web content which is too much. I’d rather use something built in but barebones rather than an addon
Where are native vertical tabs? Or at least a settings checkbox to turn horizontal tabs off.
As to why a "simple native tab group" is important even when there are umpteen extensions offering some version of it (in addition to OP's legit point about addon permissions), my take is in comment 40, https://bugzilla.mozilla.org/show_bug.cgi?id=1509350#c40 .
TL;DR: 1. Feature parity with a Chrome feature that is actually visibly used a lot (from direct observation at work), 2. Shipping a simple high-quality "baseline".
Pleaaaaase, Moz PMs & devs, some attention to this!
Done in https://bugzilla.mozilla.org/show_bug.cgi?id=1509350#c49
Vertical tabs (plus tree tabs) are also in Kagi Orion. Which unlike Safari also runs (a subset of) Firefox (and Chrome) extensions, about 70% as of their last FAQ update:
Most importantly, it lets you collapse/expand groups with just a click. Makes for a fluid built-in no-frills "putting project A tabs aside to work on project B while keeping tabs ordering and grouping, without the extra friction of bookmarks or profiles" workflow.