In response to my entry on my current Firefox addons, hwj asked a good question in the lobste.rs comments:

Isn’t uMatrix an advanced version of uBlock Origin? What’s the rationale behind using both of them?

While it's true that uMatrix and uBlock Origin have overlapping functionality (and are written by the same person), they have different purposes and focuses. uBlock Origin's focus is blocking ads and other undesired things as an out of the box experience with little configuration needed. uMatrix's focus is on exerting tight and highly specific control over what resources a page is allowed to load and use, including Javascript and cookies (and requires a lot of configuration).

(One significant difference in features is that uBlock Origin can remove HTML elements from HTML pages, while uMatrix has no support for this. Selectively removing HTML elements is extremely important for blocking ads, but it's not relevant if you're blocking entire HTTP requests. I believe that uMatrix's HTML modifications are limited to blocking inline Javascript.)

You can block Javascript with uBlock Origin and it's somewhat easier in simpler cases than using uMatrix, but you don't have the fine control over Javascript that uMatrix gives you (and this improves my experience of the web). Nor do you get the control over cookies and other types of resources, which is a deliberate simplification on uBlock Origin's part. At the same time, uMatrix doesn't give you sophisticated adblocking or things like making unwanted page elements go away, including those annoying permanent headers and footers.

So the reason that I use both of them is that they do different things for me. uBlock Origin removes ads and unwanted page elements, while uMatrix blocks Javascript, cookies, and so on. If I just wanted adblocking, element zapping, and blocking Javascript, I could probably use uBlock Origin alone, but I definitely want cookie blocking as well and I usually like the fine-grained control uMatrix gives me over other things as well.

(Writing this has given me a new appreciation for the difference between the blocklist sources included in uMatrix and the filter lists included in uBlock Origin. The blocklists uMatrix uses just list hosts, because that's what uMatrix deals with. uBlock Origin's filter lists include all sorts of sophisticated matching rules to make HTML elements disappear, as well as some host lists. Having just checked it now, I believe that all of the default uMatrix hosts lists are also in uBlock Origin, although they may not all be enabled by default.)