After installing & restarting firefox: Go to Tools> Default User Agent> Internet Explorer> Click on Internet Explorer 7.
Refresh your facebook tab now & see the changes.
Note: There's no any permanent way for disabling facebook timeline, I posted only for yours convenience. I'm using it from last 2-3 months when it came for developers & it's just awesome.
For reverting back the timeline feature: Go to Tools> Internet Explorer 7> Click on Default User Agent & then refresh your facebook tab.
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