The advice in the Mozilla Support article
Deploying Firefox with extensions:
Create the directory distribution\extensions
add-on ID for each extension that you wish to add to this directory
Add the .xpi files of the extensions to this directory, each
renamed to the extension add-on ID, while keeping the .xpi extension, or
Create in ...
Had the same problem and found a solution:
Find IP with increasing data usage in about:networking#sockets.
Then find corresponding active tab by URL in about:networking. (Filter by column "active" and use ping or something to look up the IP address.)
It was an old tab with a live webcam...