I have two gmail addresses. Successfully installed Rapportive for first while on Firefox.

Then I logged out of the first address and logged into the second. No "Rapportive" link. :(

So, I (re)installed Rapportive for Firefox while logged into the second email address, restarted Firefox, and logged into second email address. Eh, voila, there was Rapportive on the second email.

Problem solved, but WAYYY confusing.

Bonus item: Previously, I couldn't see Rapportive on second gmail when using Chrome. After this exercise, I can now go back to my second gmail on Chrome and see that Rapportive is available there.