It looks like a female Fire-breasted to me, with the buff/brown tones to the upperparts and underparts, and the relatively short bill.
I think Plain should look colder/greyer in tone and greener above, with a longer bill (slightly more like a sunbird). Compare:
http://orientalbirdimages.org/se ... _ID=&pagesize=1
Of course, they are never easy to ID, especially because they are usually so mobile!