Question:
Can web sites look into your browser, I'm using Firefox, and get information on what
web sites you have visited?
It can find out what the last site you visited was as well.
(Called a "Refer URL")
These arent things to worry about though.
The only ways someone can get into your Yahoo account:
A) They can remotely access your computer. Clear all your saved passwords if this is the case, navagate to Start> Control Panel> Add/Remove Programs and check for the following programs:
GoToMyPC, GoToAssist, LogMeIn, UltraVNC, VNC, anything with the name "Remote Access" in it.
Right click "My Computer" go to "Properties" and click the "Remote" tab, and make sure Remote Access is disabled. Remote Assistance wont be a problem, you can leave that enabled.
B) (Most likely way) They might be using the "Forgot Password" feature. You might remember, in the news, when Sara Palin's Yahoo account was hacked... someone used her "Forgot Password" feature on her email to "get in".
You can change the "Forgot Password" information in your account prefrences. I prefer put in answers that are random and inaccurate (eg: Random numbers with letters mixed in) or, a secondary password I use that no one else should know.
C) They might just know your password as well. Change your password to something that can not be guessed.
Something with both letters and numbers in it, at least eight characters long.
Make sure your firewall is updated as well.
"I have started receiving e-mails that are addressed to the person i suspect of doing this."
What kind of emails?
"I have suspected for some time that someone in the building was intercepting all of my
internet activity."
What exactly makes you suspect this? Weird messages? Strange redirects?
Also, what antivirus do you have running? Its possible that, if someone is "spying" into your computer, they may be using what is called a Trojan (as in, Trojan Horse), which is a program that "gets in" without you knowing, often looking like its something else (eg: A game). Many of these open "back doors" to your computer, allowing someone to access your computer without your permission.