At first, I saved a little effort by installing the Two-Factor Authenticator add-on so as to not require pulling my phone out constantly.
But really, i wanted this solved.
LastPass.com support suggested I purge my
~/.lastpassfolder. I did so, but without resolve.
Other sources recommended purging two lastpass folders contained in the user profile:
So i did; also without resolve.
More sources reported what I already suspected: The cookies that lastpass uses should not be deleted. So I made sure to whitelist
*.lastpass.comin my cookies cleaner. In addition to Privacy > History > Keep Until they Expire.
This didn’t work immediately, so I dug into my FireFox preferences and found that un-checking “Offline WebSite Data” seemed to resolve my issue. I had already unchecked "Cookies" and used "Cookies Auto delete" add-on to control cookies.
So in hindsight, I believe the combination of saving the lastpass.com cookies and
Edit(Tools) > Preferences(Options) > Privacy > History > Settings >
[UN-CHECK] Offline Website data
is the final solution.
Note: Upon removal/re-installation or auto-update of the add-on, you will likely be required to 2FA again. But otherwise, the 30-day 2FA memory should be retained with the above solution.