The same holds still in version 1.0.38
Two firefox-processes remain in memory (running with low CPU-usage). One has got simply "<path>\firefox.exe" as command line the other has got the very long
"C:\Program Files\Mozilla Firefox\firefox.exe" -contentproc --channel="11740.0.1299937968\2135590280" -parentBuildID 20190211233335 -greomni "C:\Program Files\Mozilla Firefox\omni.ja" -appomni "C:\Program Files\Mozilla Firefox\browser\omni.ja" -appdir "C:\Program Files\Mozilla Firefox\browser" - "C:\Users\petsi\AppData\LocalLow\Mozilla\Temp-{025607c3-1059-46dd-8b70-4c340ed01a4a}" 11740 "\\.\pipe\gecko-crash-server-pipe.11740" 1740 gpu
I disabled every extension except malwarebytes. Plugins are H264, widevine, and flash. Sytem is Win 10 Home 1809 (17763.316).
Your extension works fine during the session, but it keeps firefox busy when I want to exit the browser.