Prevent Firefox from caching localhost?

Set the HTTP Cache-Control response header to max-age=0 in your application.

And/or how to developers normally work with Firefox in this regard?

I tend to use CTRL + F5 to do hard reload (ignores cache). Pretty standard for all browsers.

Since there is no native way to configure for individual domains, you could write a Browser Extension that can intercept responses via the webRequest API (see for example onHeadersReceived) by overriding cache headers for a selected domain (you may even be able to find one already in existence).

Or simply configure settings in developer tools to ignore cache when toolbox is open:

There is also the Forget Button that can be dragged into the toolbar from customize.