It just cannot compare to the power of Chrome's/Edge's tab stacking, tab grouping and group renaming, Vivaldi's workspaces, saving tabs as sessions, tiling tabs etc. All out of the box.
Sadly, this is such a massively lacking set of features that indeed it can slow productivity in some scenarios and encourage the use of the Chromium engine in one of its many iterations.
When productivity matters, Firefox is not the first choice.
And I say that with deep disappointment, as I don’t want it to be like that. I’ve tried many extensions, but they don’t fully cover all the bases that I and the people I work with would need to reach the same productivity as with the above mentioned browsers.
Unfortunately again: "Nice, but." It's a fantastic extension that does its best to make up for Firefox's lack of tab handling. Hats off to the developer. Nevertheless, it is a far cry from what the competition has to offer. The others don't need a space-consuming side panel. They can do all these things with the tabs themselves. Firefox can do almost nothing in comparison.
To each their own, I suppose. This has been my favorite tab management system I've used.
I haven't used Vivaldi in years and only tried it for a short period at the time, but what features exactly are miles ahead or is it just the fact that Firefox requires a plugin to do some of what the competition has natively?
It also offers the sidebar on top of all the other features (although the FF plugins are a bit more powerful in the sidebars). It also lets you hide the tab bar completely, unlike FF where the sidepanel exists in addition to the the always visible tab bar... that's the definition of redundancy.
As you say, to each his own. Unfortunately, I am just that little bit less productive when using FF.
I can't speak to that 100% but I do remember trying both (albeit briefly) and ultimately settling on Sidebery.
Looking at the addons page, TST dev actually mentions Sidebery and a few others in their Developer Comments.
*Edit: It seems to imply that Sidebery is a tree addon with extra features. The most notable of which that sold me was the snapshots. I kept losing all my (~200) tabs if I closed Firefox improperly on my Macbook which was not fun.