Hello,

Try start Firefox in Safe Mode, i think this a conflict of addons:

• Firefox in safe mode

but you also can try:

• Check plugins
• Clear recent history
• Create a screenshot of the problem

This is usually caused by an extension that is using a name for a variable or function that has become a reserved variable name and thus can't be used anymore.
You will have to check if there is an update is available for your Firefox 29.0a2 Aurora version.

If it works in Safe Mode and in normal mode with all extensions (Firefox/Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit)

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes