Every tab? The zoom setting in Firefox can be set on a per-site basses.

What do you mean by 'every tab?'

Try this extension:

• <strike>https://addons.mozilla.org/firefox/addon/pagezoom/</strike>

EDIT: That should of course be:

• https://addons.mozilla.org/firefox/addon/zoom-page/

(thanks Jeff)

There's also a WebExtension-compatible extension which should continue to work in Firefox 57 and later, but I noticed it junks up any HTML you're composing (e.g., email messages, wiki edits) with font size information in all the tags. That's annoying and I don't know if there's a way to avoid it. Still, you might give it a try:

https://addons.mozilla.org/firefox/addon/zoom-page-we/