Comment 18 for bug 1168915

In Mint Mate 19.1, problem still exists. "Visit Homepage" links *are* colored and underlined, but when you click on them you get error box "Profile Missing Your Firefox profile cannot be loaded. It may be missing or inaccessible."

If your firefox is already running before you run synaptic, then the problem doesn't occur. That is a workaround but not a fix.

Apparently the problem is what Mark says above (running firefox as wrong user)