The new version has a nice-looking interface. But it is not as useful as it was.
Multiple Tab Handler used to be the single-most useful feature of Firefox after tabs themselves. Firefox is drastically less essential without it.
The thing that made it great was its intuitive operation. It was effective and powerful. It allowed you to select tabs by clicking on them as you would anything else on a desktop: using the shift key to select a sequence of tabs; and using the tab key to bring separate individual tabs into the same group. With a simple right-click you could then perform operations on the selected tabs - such as bookmark, move and close. Or you could just right-click and perform quick operations on all tabs. It was brilliant.
The new version no longer allows you to select tabs themselves. You have to open a drop down-down menu that lists all the tabs you have open. This does helpfully display the full names of all your open tabs. But this does not make up for the inconvenience of introducing further steps and waits to select and operate on tabs. It also can't handle it when you have a large number of tabs open.
You having to click an icon to open the tabs list before you can select the tabs. You may even have to click to get to the icon to open the tabs list, if you have more add-on competing for space on your browser bar. Only then can you select what tabs you want to operate on. The way it lets you select the tabs is very nice. And the way it allows you to operate on the tabs is snazzy too. It even remembers your selection if you close the list up without operating on it immediately. But by the time you get that far your bird will have flown, your flow will have juddered, your information juggernaut will have been forced off the highway for an unscheduled pitstop with the driver leaning out of the window shouting, 'what the %&$!*!?'.
Perhaps the way forward would meld the old with the new: selection as it was but now reflected in the drop down list should you need to open it, and with a working memory. Though really, would it ever even be necessary to use the drop-down list with its snazzy interface? Isn't it just superflous and therefore baggage for your system resources? How has it come to this?
MHT used to be so essential that Mozilla should have called their browser Firefox MHT. They should have called it Multiple Tab Handler - featuring Mozilla Firefox. They should have made Piro president of the Mozilla corporation, given him a big cigar and told him to just work his magic. If he had applied the magic he has done here with the MHT interface to FX bookmarks then you could have grabbed his coat tails and flown to Tau Ceti. Now they should just call it Forfux and be done with it.