Is that happening when you use a lightweight theme (Persona)?

You can try to edit the textcolor and possibly the accentcolor of the theme(s) as stored in the lightweightThemes.usedThemes pref.

The colors of the currently selected Persona are stored as the first entry in the lightweightThemes.usedThemes pref as textcolor and accentcolor. The data of other installed Personas are stores in following records in this pref in JSON format (backslashes are escaped as \"). You can edit that pref and set another value for the "textcolor":"#ffffff" part (e.g. "textcolor":"#000000").

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can try changing/creating the userChrome.css and set the font for the menubar to your liking. The following articles contains more information.

• http://kb.mozillazine.org/index.php?title=UserChrome.css
• http://kb.mozillazine.org/UserChrome.css_Element_Names/IDs

For example, yours can use:

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

/*change size, color, style of text in Menu Bar */ menubar > menu {color: black !important}

Alternatively, you can create your own style using the Stylish extension:

• http://forums.mozillazine.org/viewtopic.php?f=38&t=1570945

Be sure to restart Firefox to see your changes.

You can also add a rule to disable the text-shadow in addition to setting the (text) color.

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

#navigator-toolbox .menubar-text { text-shadow: none !important; }

• https://developer.mozilla.org/en/CSS/text-shadow