Resolved how to change the size of the font in the menu-bar?

userChrome.css can change it.

 

Here's mine. Try it and see how it looks. You can edit the font-size, font-weight and font-family. A font-weight of 600 = the first level of bold.

Code:

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url( "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul "); /* set default namespace to XUL */

/* menus */
menubar, menubutton, menulist, menu, menuitem, .tree-rows {
  font-family: Trebuchet MS !important;
  font-size: 3.7mm !important;
  font-weight: 550 !important;
}

/* hide toolbar borders */
menubar, toolbox, toolbar {
border-style: none !important;
}

 

For Bookmarks Toolbar use these 2 and change the font-size to your liking and change the width & height of the toolbarbutton-icon to your liking.:

Code:

#PersonalToolbar .bookmark-item {
  font-size:18px !important;
}
.bookmark-item:not(#bookmarks-menu-button) > .toolbarbutton-icon {
  width: 20px !important; /* 25% over 16px default */
  height: 20px !important; /* 25% over 16px default */
}

Education:
CSS font-size: http://www.w3.org/Style/Examples/007/units.en.html
The !important declaration gives the user's styles (your styles) seniority over the author's styles. For example, on a Webpage the author may declare fonts to be a certain size but your browser settings or userChrome.css can override them. And similarly, the FF install has default sizes for its interface and menus, and your userChrome.css can override them too. Thus the use of !Important so as to always override the defaults.