Not your average addon, but incredibly powerful and versatile once you learn how to use it. Definitely recommended for advanced users, the learning threshold might be off-putting to some, but it rocks once you get the hang of it

One of the must-have addons. It blocks all kinds of contents (cookies, CSS, images, scripts, media, frames and etc). You can control which 3rd party sites can load content. I cannot surf the web without uMatrix.

This is a fantastic tool for keeping unwanted, unnecessary javascript from running on web pages. The fact that it works in Firefox for Android is the main reason I only use that browser on my phone.

Thank you Raymond Hill and good people at Mozilla!

This add-on is amongst the vital add-ons that brings to Firefox users the joy and efficiency of navigating the web. It molds very good to 2 principles that made FF the most powerful(at least before FF v.56) browser : open source(code is open so anyone can take a look at it, and if someone wants to modify, it can fork it respecting the licence) and privacy(respects and help the user when it wants to have at least minimal privacy on the web).
I have learned a lot about how webpages work, using this wonderful add-on. For me, the more functionalities, the merrier!
I have only 2 things that bother me when I use this add-on.
First and the biggest is the fact that I cannot Exclude a few domains while umatrix is enabled for a webpage. What do I mean: when opening a webpage some domains(primary) are green(allowed) and some are red(blocked); say for ex. 4 green and 30 red; I want to keep filtering Enabled but block(make red) only 3; to do this I have to click 1 or 2 times on each row to make them green; that is some 2x27=54 clicks. MY SUGGESTION: while filtering is Enabled, make all rows green(clicking a button or something) and then click only on those rows that you want to block(make red).
<<< Edit ! I see the new * button"Select the global scope to see/create rules which apply everywhere" ; that way I can block a domain everywhere I go.
Still my first point, I cannot see how it can be done in this 1.3.14 version.
!! Would be excellent to be able to whitelist all, then without doing a refresh, to blacklist a few domains while still having the matrix filtering enabled. !!
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Cheers !

<<< Edit
I regularly use a good olderthan56 version of FF, and wrote the review now, without testing the latest addon on the latest FF. After I have tested the latest version(unfortunately it does not work on FF older than 56) of this wonderful add-on, one of the things that bothered me is gone (The second thing is that I cannot make the text bigger( while still umatrix show me all the table when I scroll down). I have already set the Text size to Large in umatrix settings, but when sometimes I stay at 2meters(6.5feet) from the monitor I cannot see the text in umatrix, while I am able to see the text on the Operating System(where I have set the "text scaling factor" to 2.2) and F.F.(where I have set in about:config the "layout.css.devPixelsPerPx" to 1.5).)
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The best.Thank you for all your work

This is the privacy/security extension I have always wanted. Complete control over what gets blocked with rules which can be whitelists or blacklists, global or local, with an interface which makes specifying complex rulesets as simple and intuitive as is possible.
It requires some tinkering to get all the functionality you need from the web without loading any unnecessary trackers but once you get a good set of rules in place for the sites you use, this is the ultimate *-blocker add-on.

good