All the time, some of the time . . . .

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

You would have to avoid hovering the map (move pointer to far left or right) if this map supports a feature to use the mouse wheel to zoom the page in and out. There is no way to prevent this otherwise.

I restarted both computers in Safe Mode: a Windows 7 machine with a mouse and a Windows 10 machine using the track pad. Both with Firefox 55.0.3.

Each zoomed on a map once the cursor hovered over it. What a pain in the neck.

This is recent behavior (about a month ago).

As cor-el mentions, content in a page can intercept mouse and gesture actions and interpret them as they wish. Make sure your mouse pointer is positioned over a blank area of the page when navigating to get the default behavior.