Hey there, let me explain the situation:

• I have no apps running in the background
• I'm not connected to Wi-Fi or 3G
• My email interval is set to manual
• Widgets on my desktop don't update automaticly
• The screen was off all the time and device was completely IDLE
• I usually use killall browser, browserd to prevent it from running in the background
• I have not installed or updated any application recently

Yet my battery was draining at a very fast rate like the picture shows below:

Any suggestions what could the problem be?

Defective Battery? - have you only noticed this once? (how old is your device?)

Old simcard?

Needs more information.

Install htop, and on the console enter:

htop --sort-key TIME

See what's at the top, and how much time it's used.

Well it's either your battery or a program is running and you're not aware?

Look if this happens again, and check the time it happens.
If it is the same time, then it might be something deeper.

browser and browserd are running all the time, and are unrelated to fennec.

every once in a while browserd goes haywire (probably the worst offender) and usually does so shortly after a period of use, particularly after touching a page running Flash.

htop's smallest interval counts in 100ths of a second, had me confused for a bit. browserd using 6 hours of cpu time isn't unusual if you're using it and leaving it in the background. If you hit another power draw like that, take note if the device feels warm and grab the times like you just did. Also note if a page you visited happened to have Flash on it, since that seems to be when the browser behaves the worst for me.

If you install the BatteryGraph you'll be able to check network and CPU usage together with the battery level. It's quite similar to the battery-eye you're using.

Good luck.