A great addition for this extension would be new tab buttons alongside the default button. I've attached a quick-and-dirty mockup of how I envision this.

This is cluttering the UI, especially if you have a lot of containers. What鈥檚 wrong with the long press? Excepted that it lacks discoverability.
This is cluttering the UI, especially if you have a lot of containers
Excellent point - I didn't think about what would happen if someone had many containers.
What鈥檚 wrong with the long press?
As a user, UI delays are annoying (when they only exist because of UI design). Honestly, for many workflows, the long press works great for opening specific containers. I've found that I typically close tabs when I'm finished with them. When I'm performing tasks that require multiple tabs open at the same time, I find myself long pressing the new tab button often enough that the long press feels like it's slowing me down.
I think many users will have only a handful of containers that they use often. It would be nice to have the ability to add container specific new tab buttons via the customize interface.
OK. Drag&Drop colored new tab icon for selected containers seems nice: this way, you can add only the most relevant one there, and the rest of them are still under the long press.
This is a cool idea, but not possible from the platform & Firefox UI.
A separate addon could build something like this, but the platform will restrict you to only adding 1 extra icon (1 extra plus button, but not multiple).
@groovecoder What makes this not possible?
@jonathanKingston can explain better. There's currently only a way to add 1 additional item to the tab bar UI. To add more, we will need the feature in mozilla-central, and Firefox UI team wouldn't approve adding more clutter to the tab bar UI.
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OK. Drag&Drop colored new tab icon for selected containers seems nice: this way, you can add only the most relevant one there, and the rest of them are still under the long press.