I can't read the text on my toolbar after changing personas, and unchecking "use text color" doesn't do anything even if I change personas.
Several personas come with unreadable text and accent color. This article explains how to resolve the issue: http://support.mozilla.com/en-US/kb/Cannot+read+text+after+selecting+a+Persona However, when I followed the directions, the text colors did not stay the same as I switched personas.
If you have installed a persona then the default text color and background color of the toolbars are defined by that persona.
The colors of the currently selected persona are stored as the first entry in the pref lightweightThemes.usedThemes as textcolor and accentcolor.
You can edit that pref and set another value for the "textcolor":"#ffffff" part (e.g. "textcolor":"#000000").