Locate the addon you installed that provides that weather feature.
https://addons.mozilla.org/en-US/firefox/

Then look for instructions on that AMO page or look for a "Help" link.

Firefox has no built-in weather toolbar so this will be part of an add-on.

Try right-clicking (or Control+Click) on the weather icon in the toolbar to see if it brings up a way to change the settings. If that does not work, open the add-ons manager (in the Tools menu select Add-ons). In the list of extensions select the weather extension then click on the Preferences button. that should hopefully open the preferences dialog giving you the option to change the location.

After fussing around for a good half-hour, this was the easy solution that had eluded me. Control+Clicking brought up the options I was looking for!

Thank you so much!