[seodisparate]

Last I checked, uMatrix by default enables everything for 1st party sites. So if you are at twitter.com, all stuff originating from twitter.com will be allowed.

You should be able to remove this rule in uMatrix's settings in the "my rules" tab. Just click on the rules you want to remove on the table on the right and click on commit to save your changes.

[SAI_Peregrinus]

You can also just click the box for scripts of a given site to turn it red (blocked).