Hi, you can go into Customize to the bottom of the page and will find Themes, can change to dark there. You can also search for other Dark or Black themes here you may like : *https://addons.mozilla.org/en-US/firefox/search/?platform=mac&q=black *https://addons.mozilla.org/en-US/firefox/search/?platform=mac&q=dark While in Customize you can drag out to the top bar the icon that has -|100%|+ out and will take awhile to modify all your regular pages you visit but will be under your control and it will remember what to do on each page. There is also this : *https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/ The above can change the size of font menus in Firefox. Can not do anything about what websites do with their sites that will not allow it. *https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use

Maybe you'd like to give this a try : https://addons.mozilla.org/en-US/firefox/addon/nosquint-plus/

You can also ask advice at a Mac oriented forum to see whether your Mac OS version has builtin accessibility features that might be helpful.