What site or App are you using for this?

Hi, is this on the built-in Firefox Home / new tab page?

Actually, I think Firefox only remembers one location at a time, so it should not reset automatically.

If the setting is lost between Firefox startups:

Normally, your preferred location is saved along with other settings in the prefs.js file so that they can remain stable the next time you exit Firefox and start it up again. If settings get lost between sessions, some reasons are described in the following article:

How to fix preferences that won't save

If the setting is lost when you leave the new tab page:

Do you only have one copy of the page open? In other words, there aren't any other new tab pages open with a conflicting setting?

I can't explain this, unless somehow your Firefox got locked to a specific location. Maybe there are some settings to check if we think about it.

What site or App are you using for this?

In Firefox https://support.mozilla.org/en-US/kb/get-your-local-weather-forecast-firefox-address-bar