What's weird is with Firefox for Android it's so difficult erasing a site from being remembered. Once visited, in my experience, even after deleting the history entry and last closed tabs item it still auto completes the domain in the addressbar (when it didn't before) and the only workaround is a full history wipe (since the Android version offers no granular timeframe like the desktop version).

So if accidentally clicking some link from some other app that auto opens the default browser it's a PITA to get FF for Android to forget about it.