There are minor benefits without ABP. For each page/iframe that is created, we can avoid running the CSS cascade (and share those data structures) for the UA style sheets. It's saves something like 100 KiB per document.
If other add-ons insert a common style sheet into all documents (it's plausible Stylish does something like this? though I've never checked) then we'll again be able to avoid running the cascade and having duplicate data structures for the cascade across different documents.
If other add-ons insert a common style sheet into all documents (it's plausible Stylish does something like this? though I've never checked) then we'll again be able to avoid running the cascade and having duplicate data structures for the cascade across different documents.