HI   ! Instead of using an add-on, could this be an alternative for you ? https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use

I think the "Theme and Font Changer" extension is more directed toward the user interface than toward pages. Is there a problem with particular font faces that are hard to read, separate from the issue of being too small? If there isn't already an add-on that lets you modify these individually, by site (for some or all of their fonts) or by font (for fonts you never want to see anywhere), there should be. It seems that Chrome has at least two extensions like this: * [https://chrome.google.com/webstore/detail/font-changer-with-google/jgjhhoglgjdklldfgoffdiaceffijeke?hl=en Font Changer with Google Web Fonts] * [https://chrome.google.com/webstore/detail/font-swap-for-google-font/onhgmcpflfncjkdbclmhnfondmindgbg?hl=en-US Font Swap - for Google Fonts] Over the past year, Firefox has been building nearly complete compatibility with Google Chrome extensions, requiring authors to make only a few changes (in most cases) to convert them to Firefox extension. Maybe one or both of those authors can bring their work over to Firefox soon?

You can look at Zoom Page (WE) extension to set a default page zoom on web pages. *https://addons.mozilla.org/firefox/addon/zoom-page-we/ *https://addons.mozilla.org/firefox/addon/zoom-page See also: *https://www.ghacks.net/2016/05/23/install-google-chrome-extensions-firefox/