==The TMT arrow was not working for me and it turned out it was due to me having 'double click' mapped to 'Duplicate tab' on Tab Mix Plus, so I set it to do nothing on there and then the TMT arrow worked normally with a double click==
I haven't tried every single feature of TMT, but so far it works great, specially since I had so many tabs from different categories. There are just a couple of issues that are always in my head bothering me:
The first one is that I really dislike the big yellow pin icons for the pinned tabs and I can't change or edit them. I wanted to edit the Pin, List All Tabs and Closed Tabs icons to be more like Firefox's current simple white icons, but I don't think they can be edited manually ("unsigned add-on"). If not that, I wished there was an option to not mark the pinned tabs with an icon and have the unpinned tabs marked with icons instead (since there's an option to pin tabs by default, which I chose). The icons also mean less space for the tab names/titles, so there's also that.
The other issue I have is that it would be so nice to have an option to set the tabs to be grouped in columns along the TMT toolbar, rather than in rows (kind of like the rows switching places with the tabs in the TMT toolbar) and have the Tab List of each column/group appear above each one when I click on them (like they appear when clicking on the 'List All Tabs' button, but for each column/group). I have many rows right now and I don't want to have more than one visible on the TMT toolbar because I don't want to take all that space away from the page.
The notion of not having these options is always itching, but I know my issues could be considered more like a matter of preference, so I'm not sure if I should list them here...