I assume you mean the Gmail app, and not the Gmail website in Firefox; I'm not sure the website is possible.

On Android, the default applications are stored at the OS level, and Firefox should follow that. This article describes where to look in settings to clear the default from the Mail app (i.e., look for Mail and not a browser): Make Firefox the default browser on Android. Does it work?

Thanks for the swift reply. Unfortunately, I'd already tried that. The problem is that the list of apps doesn't include the default android email client. That is, this seems to be internal to android rather than something else running on my phone. I don't even currently have an email account set up on the phone because I do just want to run Gmail (which is working fine for checking my email).

I do mean the Gmail app, by the way… Any other thoughts?

You should be able to find Email as a separate items. Make sure when viewing the Apps list you're in the All list and not the Downloaded list, or you won't see the pre-installed apps.

There is a drastic option to reset all defaults. Above the Apps list, tap the "3-dot" menu, then "Reset app preferences". I haven't ever tried that myself, for obvious reasons, but have seen it suggested on forums when there are problems with setting or clearing default apps.

In case it's a Sony thing: https://talk.sonymobile.com/t5/Xperia-Z2/bd-p/xperia_z2

Thanks! Viewing "all" apps - which required scrolling the screen sideways - and then resetting the defaults of the "email" app fixed it. Props!