Is it possible that somebody who has access to my laptop can flash a new version of bios which does spying too i.e. recording keylogs, url browsed, downloaded files etc and send it to remote location and i will not even come to know about it ? I am running ubuntu 13.10 on dell inspiron.
What you're referring to is theoretically possible (i.e. malware which replaces your computers BIOS keeping it's operation intact but adding malicious functionality), but it is very non-trivial to pull off this kind of attack.
The attacker would need to be targeting you specifically as there's really no such thing as a general BIOS that they could place onto the machine. So they would need to custom craft something for your specific model of laptop, then likely get physical access to it to install it.
That said, done correctly this attack would likely not be detectable by standard user tools, although in depth forensics should be able to notice the difference between a standard BIOS and the one installed on your machine.
If you're interested in reading more on the difficulties and possibilities of this kind of attack lookup "BadBIOS" on your favourite search engine.