1. You are using an older version of Firefox, you should update to the latest release - Firefox 22.0. Help > About Firefox and use the Check for Updates button, if the update doesn't happen automatically.
2. Exactly what did that message say? There is no "safe reading mode" in Firefox.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).
Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.