|If Firefox was already installed, the Registry keys already exist under your profile. That's why it's not seeing this as a new install, thus, no prompt to import.|
What you could do is delete the Favorites, then use the Import option to bring them into Firefox.
"So won’t you give this man his wings
What a shame
To have to beg you to see
We’re not all the same
What a shame" - Shinedown