One of the most valuable features of older, correctly-functioning versions of Firefox was the capability to right-click and rename tabs.
This was invaluable to me. Now I am forced to use older Firefox in secure VMs to be able to have access to this feature as I cannot easily do without it. Having this capability meant that if I opened a tab intended only for search, I could rename this tab to “Search: blah site” and I’d know always what I was using it for.
When you have hundreds of tabs open, this is really very useful and I am robbed of hours every week where I do not have this functionality to rename tabs.
All told, losing this and all the features to the new pseudo-secure Firefox means that I now need to spend 2-3x as long to do the same things I used to do.