blocking url using firefox extension

How can I control access to webpages in FireFox. I am going to develop a new addon.

Does FireFox extension programming allows this or should I develop a desktop application to control access.

edit: It is not going to be used daily. Will be just an example for rese开发者_高级运维arch.

I want to develop an application/extension that will start to work if a user wants to open a webpage. Controls webpage using a webservice, developed before, then open requested webpage or deny and say this webpage cannot be accessed.

Firefox addons can always be disable, user could use another browser. Better to use a desktop app. IMHO.

Depending on what you're trying to accomplish, and what the environment looks like, you could for example:

1. Use a custom filter with Adblock to filter out certain URLs
2. Use a filtering HTTP proxy such as Privoxy
3. Use a firewall to block certain remote servers