Even though Flash is disabled, this site plays three video ads and isn't finished loading after three minutes and 350 requests. All for 15 paragraphs of actual content. Horrible.
Annoying as it sometimes is, I'm starting to love uMatrix.
A couple of tips for your ruleset:
* disable first-party plugins (flash, PDF)
* enable first-party "other" (html5 video, audio)
I also enable various CDNs globally, i.e. scripts from cloudflare cdnjs, and frames/video from youtube and vimeo. Though I don't really recommend this if security/privacy is a concern. At least they won't get your cookies :)
Edit: oops, just noticed you accept first-party everything. Still, disabling plugins should prevent a wide class of exploits (see recent Chrome PDF vulnerability).
I've learned some interesting things with uMatrix. Like, on Twitter page, the password field of the login box doesn't work unless you let the site download some script (the username box works fine)...