Disable TLSv1.1 and TLSv1.2 in the Firefox helper

With legacy/trac#11253 (closed), Tor Browser's Firefox config has TLSv1.1 and TLSv1.2 turned on. If meek-http-helper (browser TLS camouflage) sends Firefox 24 ciphersuites but uses TLSv1.1 or TLSv1.2, then it will look weird, because as I understand it, mainline Firefox 24 has TLSv1.1 and TLSv1.2 disabled. ([[doc/meek#Sampleclienthellos]] corroborates that ordinary Firefox 24 uses TLSv1.0 when connecting to Google.)

We also need to remember to turn TLSv1.1 and TLSv1.2 back on when they get enabled in the next ESR...