Very minimalist and precise.
Suggestion: I could give you a prompt to tell that the tab is being closed by the add on, just to make sure that is not a bug on my browser.

Works perfect for me because of its precision

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I have checked many extension available but this is the best one with simple whitelist and black list solution. But for proper usage this extension need improvements.
1. Need password to restrict other users (except admin) changing the settings.
2. Updates to the whitelist and blacklist done via the toolbar-icon should be removed from there and changed to option setting page with password to avoid option changing.
3. My aim of using whitelist and blacklist via Firefox was to help my kids use internet effectively and this is the only simple solution i have found. But need modification otherwise my kids can easily change settings.
4. Add timer to restrict access so this will be a complete a parental tool.
If you can make these changes I can give you thousands and thousands of users. Thank you.

Under all these Blocker Addons, this is almost what I was looking for and also the most precise and best working one out there.
There is just one thing that still annoys me. When you are on a website and (accidentally or not) click on a blacklisted link, it closes the whitelisted tab you are on with it instead of just not opening the link in the first place. Not opening blacklisted links should also be a thing in general to not trash the history with useless links.
The two text fields could also use a bit more height - there is a lot of free and unused space below the save button.

Just want to thank you for this. Does exactly what I was looking for. I want to improve my productivity on the internet and this does it for me.
For example, I block 'youtube.com', but I allow 'youtube.com/feed/subscriptions' and 'youtube.com/watch/*', this means that the only pages available to me are the subscriptions page and the videos that I subsequently click on from that page. No more distractions.
(ofc I can still view random videos if I click on youtube links from outside youtube, but this addon stops me loading up youtube to see what's new and then getting sucked into watching things that I'm not even subscribed to).

Exactly the level of precision I needed. Thank you so much.

Works perfect!

This is the best website blocker. totally suits my case where I wanted to allow only specific set of websites to use. Loved it's simplicity

This is the only extension I've found that allows both a whitelist and a blacklist to run at the same time. Really helpful. I just wish it was open source so I could help with some of the TODOs.

This should be password protected, One main reason of whitelisting something is to limit the site access for others using / sharing the computer, and if anyone could change whitelisted sites, then the purpose is lost.

The author fixed a bug where I did not see the icon on the toolbar. It blocked sites for me, but I am using it in conjunction with PublicFox to lock my settings from being changed. PublicFox will not lock this since the settings are accessible in the toolbar. So I ended up using Simple Blocker, which worked to enforce a whitelist for me.