You can set the size of all UI fonts (or the UI font globally) using userChrome.css.

Is this about bookmarks on the Bookmarks Toolbar or in other places like drop down menu or the sidebar?

For the Bookmarks Toolbar this would be this code in userChrome.css.

You can add code to the userChrome.css file below the default @namespace line.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#personal-bookmarks .toolbarbutton-text { font-size: 15px !important; }

See also:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• toolkit.legacyUserProfileCustomizations.stylesheets = true

See:

• https://www.userchrome.org/firefox-changes-userchrome-css.html

Since Firefox 57, extensions are not able to modify much about the toolbars.

Would you be interested in a global increase affecting text on tabs, in the address bar, and on the web, as well as the bookmarks? There is a built-in setting for that. If you want to try it, here are the details:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box in the page, type or paste devp and pause while the list is filtered

(3) Double-click layout.css.devPixelsPerPx and change its value to 2.5 then press Return or click the blue button to change the change. Note: the default value is -1.0 but do not use the - on your values.

If you have a Retina (double-resolution) display: The text should enlarge by 125%

If you have a standard (single-resolution) display: The text should enlarge by 250%

To make this more useful, you can experiment with other values using the same method: double-click, edit, save.

Please be careful not to select too low a value to avoid everything becoming tiny. If that happens, hold down the Command key and tap the + key several times until you can see the Reset button (arching left-pointing arrow at the right end of the bar) and click that to return to normal. Then hold down the Command key and tap 0 (that's zero) to reset the zoom and you can start again.

Can you get it to a size you like?