The Search By Image (by Google) extension causes very high CPU utilization during regular browsing, often damaging user experience significantly, in a way that is very difficult to associate with the extension. We gave the developers advice and sufficient time to correct the issue (see bug 777266), but so far only minor performance fixes have been issued. Since its usage stats are worrying, we have decided to go ahead with the block, which will be pushed live this Friday.
The block has been staged now: https://addons-dev.allizom.org/en-US/firefox/blocked/i117
I remember using an older version of it but the cpu was just fine, then later, i updated it to 1.0.6. At 1st, I thought it was my comp or Firefox using up to 50% CPU but it was Google's newer add-on, 1.0.6. Firefox was slow overall. Please tell Google about this. The 1.0.4 is the safe version. Test it out. ;]