There is a completely safe and secure method of blocking browser (and other) access to ad and other undesirable sites.
Create a custom hosts file which re-directs access requests to for example dodgysite.com to your own PC. The PC knows that it can't service the request and quickly rejects it.
Obviously someone has already prepared one.
Using a Hosts File To Make The Internet Not Suck (as much)
I have not tried this myself although I have used hosts file in the past. I suspect that it won't work if you are running a web server on your PC because the requests will then be passed on to it.
It is safe because it uses no code and is a text file that can be inspected.
There is a chance that your corporate IT people are using the hosts file for their own purpose. Not that likely nowadays but you should check. Just have a look at it. You can always simply append your entries to their ones.
On my Windows 7 PC the hosts file is blank other than some comment lines (begin with #).
Some viruses use the hosts file to redirect innocent web requests to other sites and so it is usually only possible for an admin user to edit it. It is best left so that only admin users can edit it. Do not let this mention of viruses put you off from the method. You are using the same method but for a different purpose.
Writing here has convinced me that it is such a good idea that I have just installed the above listed hosts file.