Advanced: install as a userscript

A userscript runs inside a userscript manager add-on instead of being its own add-on. It needs no signing and also works in Chrome, but it does not remember your chosen size per site and has no toolbar button — just the on-page control.

Firefox on a computer (or Chrome)

Install a userscript manager: Tampermonkey or Violentmonkey.
Download text-size-adjuster.user.js — most userscript managers offer to install it as soon as you open that link.
Drag the file into a browser tab, or use the manager's Utilities → Import from file, and confirm the installation.

Firefox for Android

Install a userscript manager that supports Firefox for Android — Tampermonkey has an Android build; check addons.mozilla.org from your phone for current availability.
Open the .user.js link in Firefox for Android (or use the manager's import option), and confirm the installation.

After installing, the round −/+ control appears in the bottom-right corner of every page.

Settings

Open your userscript manager's menu (its toolbar icon) while on any page: Text Size Adjuster adds commands there to place the control in a different corner, or to show it only after you zoom in. Your choice is remembered.

If you hide the control with ×, the show command in the same menu brings it back; so does loading the page again.