I enjoy the grouping functionality, but it has some significant performance issues. Whenever a group/folder is opened, it seems to automatically switch to whichever tab was last active in it. Even if you click away or do something else before it finishes, it will drag you kicking and screaming to that one tab. Sometimes, it will be very slow in this regard, only switching to said tab well after I've already left the group and collapsed the folder, and this can also happen with two groups at once, helplessly dragging me back and forth between two tabs. I don't know if this is an intended feature, but please add an option for disabling it.

A good approximation of Chromium style tab grouping. There are issues, but I assume that's based on the way Firefox itself is built and limitations in circumventing those designs. It essentially does what is says on the tin, but it's moderately to extremely janky. Hopefully it will get smoothed out in the future.
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One of the more impactful/frustrating issues I've run into (and the work around) is:

After closing a browsing session and reopening Firefox, one of the windows' tab groups no longer displayed, however tabs continued to hide/show themselves according to the invisible group's functionality. Unfortunately these were a group of tabs I intended to work through.

In order to access any of these hidden tabs, expand the tab menu (the inverted carat to the right of add-tab button). Go to hidden Tabs and select one. Since the Tab Group's logic still functions, this opens the selected tab and all others that were in it's group. However, the group doesn't appear so leaving the group will cause the issue to persist.

Shift+Click to select the tabs in the malfunctioning group. The Right click and add them to a new group. This new group will be visible and functional.

Kind of a pain that this is necessary, but it beats not having tab groups at all