hello jjjhornet, i have the same issue every now and then & didn't find any real solution for it yet either...

i'm mainly using the aurora build, can't remember exactly for how long this ongoing but it occurred at least since the 19a2 builds. clearing the various plugin related files within the profile folder didn't bring success :-/

as a workaround i can launch firefox into safemode, then from within it select "restart with addons disabled" and after entering safe mode again disable plugins - their state will then be usually remembered after a normal restart of the browser.

edit: another user reported the same issue here.

A possible cause is that the pluginreg.dat file is regenerated,so make sure that you do not have cleanup or security software that removes such a file.
I don't know if the presence of plugins that are blocklisted can cause this to happen or a possible update of such a blocked plugin that would enable it.
With a Nightly (21) build as used in that other thread, all things are possible.

i don't use any cleanup tools & MSE is my security software, which is rather unobtrusive and shouldn't interfere here...

i currently have one blocklisted plugin (see screenshot), but if i remember it correctly the problem was already present in my case before i had the outdated acrobat plugin installed.

sometimes when you enable/disable a plugin in the addons manager & press f5 afterwards, it immediately switches back to its old state/sometimes it will stay on the new state until a restart of the browser & in some cases it will remember the state even across sessions - it's just not working reliably in any way.

as far as i can observe the pluginreg.dat file in my profile folder the change of a plugin's state isn't written to the file as soon as it's toggled in the UI during a session, but only when the browser is getting closed - so the information has to be in the memory somewhere before and might be getting corrupted. i suspect it might be a general bug but i cannot reproduce it reliably .

edit: forgot to attach the screenshot & i'm using the current aurora build

i don't know if this is making any sense - but when i go into my profile folder (firefox > help > troubleshooting information > profile folder/show folder) and delete the sessionstore.js & sessionstore.bak files the problem seems to go away.

edit: i've narrowed it down to a point where a change of the plugin state isn't retained accross the restart of a browser session when a gmail-tab is active in the background while you toggle the plugin on/off

edit2: filed bug #839728

Bang on! The problem for me was a facebook tab and facebook video calling plugin.

Brilliant response.