I closed that bugged window, leaving an 830-tab environment for subsequent test sessions.
I began a session with all extensions disabled. After enabling the extension, Firefox was almost completely non-responsive for around twenty seconds. Two frames from a 150-second recording:
– playback is accelerated in places (see the clock, bottom left, speeding) but truly, the delay was around twenty seconds.
Following a brief discussion in IRC I planned to copy the root directory and cache directory of the affected Firefox profile to a Tier-1 platform – an old MacBook Pro where Gecko Profiler is installed, and appears to usable, with Firefox 56.0 (64-bit) on pre-release Mac OS X 10.13.1.
OT (thinking this through): some testing with that particular Mac, where USB is limited to 2.0, is hindered by my use of Apple File System for the startup volume on a hard disk drive. if you’re interested in performance issues with APFS, follow https://framasphere.org/tags/apfs – there’ll be more from me when things are less busy. Back on topic …
It’ll probably be easiest for me to test data from the affected Firefox profile with 56.x on 10.9.5 (Mavericks).