Heres a couple things to check:
Make sure your Removal settings are active, and Cookie is Enabled for Automatic removals. Look on the Removal >> Removal Options tab - Cookie will show Pause Automatic Removals if it is active - press the button to enable if it shows "Enable Automatic Removals"

Cookie doesn't display "session cookies" either - which can be a cause for discrepancies. Session cookies are cleared automatically by the browser when you restart the browser.

what are your Privacy settings in Firefox?
You should use Firefox will Remember History
Or if using custom settings Keep until they expire