Tabs do not react to the drag-start event when I try to move them right or left
on the tab bar.

this also affects
1) Tabs on the QuickFolders bar
2) entries within the subfolder menus (which pop up when a tab is clicked) -
these could previously be dragged do create new tabs.
3) the "current mail button" - this little envelope on the left hand side of
the current folder bar (in the middle of the screen) could previously be used
to drag the selected mails / conversation to a different location

(In reply to comment #0)
> Tabs do not react to the drag-start event when I try to move them right or left
> on the tab bar.
> 

All drag breakages after QuickFolders 4.6 are caused by 

[Bug 1226362]
https://bugzilla.mozilla.org/show_bug.cgi?id=1226362

which ultimately comes from the final deprecation of the Mozilla specific
events
https://developer.mozilla.org/en-US/docs/Web/Events#Non-standard_events

draggesture 
dragexit
dragdrop 

to fix the bug, these need to be replaced with their standard counterparts

dragstart
dragend
drop

Created an attachment (id=8252) [details]
4.7 prerelease 56

This bug is going to handle all drag+drop breakages in Tb50 and Tb51. The
attachment was tested with 51.0b1 (32-bit) and works fine with this.

(From update of attachment 8252 [details])
Note that for some reason the install.rdf of this XPI includes
em:version="4.7pre55" i.e. one prerelease number below what the attachment
description and XPI filename are saying.

Resolved in 4.7 release