@firefoxnightly the problem is actually far more severe* than i initially suspected, & alas still not fixed... to the extent that atm & since the bug appeared,
is actually unusable for me... so i had to spend a day temporarily(?) abandoning it for boring vanilla orange [yuck to that colour]
, by creating replica profiles for all my old Nightly ones in it.
* whereas i initially discovered the bug when in Customise Toolbar... mode, in fact it occurs throughout
the browser, eg, stuff also now impossible includes rearranging bookmarks, rearranging tabs [ergo now mouse-based tab stacking et al is impossible].
over the many years i've used Nightly as my daily, the online naysayers keep talking it down as being too unreliable for daily use. my longterm experience with it proves that to be #FUD, nevertheless it is true that rarely, maybe every few years, something severe enough does arise that briefly stops me being able to use it. it seems that cycle is now once more at its nadir.
#FirefoxNightly 
Droppie
Firefox Nightly