I noticed lately i have been using my left thumb on the fret board when playing a note on the e-string. Is this a bad habit?

Yes, VERY bad. It forces your left wrist into a really bad angle, and if you do it often enough for long enough, it can lead to damage or injury. One thing in particular that you will have to look forward to from doing that would be carpal tunnel syndrome (CTS). I have it in my right hand (non-playing cause) and it's not fun.

Your left thumb should be behind the neck, the tip on or near the middle of the back of the neck. If you do this, and don't have the bass hanging around your crotch :eek:, your wrist will be fairly straight, and you will lessen your chances for harming yourself. Also, it lets you get all 4 of your fingers on the fingerboard, eliminating the need to use the thumb to fret any notes. Hope that helps.

Ditto... my little brother started playing like this and it took me months to get him out of the habbit. As soon as he got used to playing normally he could play faster and more complicated pieces much easier.