Note that personal-bookmarks is an ID and thus needs "#" as prefix.

• https://developer.mozilla.org/Web/Guide/CSS/Getting_started/Selectors
• https://developer.mozilla.org/Web/Guide/API/DOM/Locating_DOM_elements_using_selectors

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

#personal-bookmarks .bookmark-item > .toolbarbutton-text { font-size:12pt !important; font-weight:bold; }

Note that this code only works for the Bookmarks Toolbar. If you need to increase the font in other places as well then consider to use the Theme Font & Size Changer extension.

• https://addons.mozilla.org/firefox/addon/theme-font-size-changer/

As you see, I am no css person. Have gained TINY knowledge about it over the last couple of years--and I do mean 'tiny.' I thought the # was a "whole line comment" mark and removed it.

With #, it sill doesn't work. Below is what I have. It is in C:\Users\[name]\AppData\Roaming\Mozilla\Firefox\Profiles\2h0sftlb.default\userChrome.css

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

#personal-bookmarks .bookmark-item > .toolbarbutton-text { font-size:12pt !important; font-weight:bold; 
}

Oooo system turned the # int an indented "1" I don't see a control to enter code????

Do you see what I am doing wrong?

You need to create a chrome directory and place the userChrome.css in that folder.

C:\Users\[name]\AppData\Roaming\Mozilla\Firefox\Profiles\2h0sftlb.default\chrome\userChrome.css

cor-el said

You need to create a chrome directory and place the userChrome.css in that folder.

That was it! Thanks! What would be the element spec to manipulate the text in tab labels?

You can use this code for the tab label text:

.tabbrowser-tab .tab-text { font-size:12pt!important; font-weight:bold; }