I guess the problem here is there is no way to cache this css injection. Maybe FF devs should do something about it instead of just whining about how an add-on is unable to handle the browser's shortcomings.
We could say the same about jQuery or bootstrap. You could have pages with many iFrames to other pages using the same js or css files. Right now these pages waste memory on the browsers too...
It's possible to make this case better in various ways. Pretty much all of those ways involve tradeoffs that increase memory usage or the amount of CPU time needed for CSS processing on normal pages without ABP involved.
It's not clear that we want to make those tradeoffs.
We _should_ of course make the changes that don't involve such a tradeoff. They're not trivial, and have significant opportunity costs, so haven't been the top priority for Gecko, unfortunately. You can tell how long https://bugzilla.mozilla.org/show_bug.cgi?id=77999 has been around, for example.
Just to be clear, what we're talking about here is a stylesheet which has about 13000 rules. The data structures used inside Gecko to optimize dynamic changes to documents without having to recompute all style for the document are about 200 bytes per style rule. So you get about 3MB per document, which can add up once you have a large number of documents. Ignoring ABP, having that many rules is not that common, and having that many rules in random dummy iframes sprinkled in your page is something that basically never happens.
We could say the same about jQuery or bootstrap. You could have pages with many iFrames to other pages using the same js or css files. Right now these pages waste memory on the browsers too...