My preferred uMatrix rules to start out a new profile, block pretty much everything, including 1st party stuff. Only allow CSS and Images to make it readable before enabling, scripts, cookies, etc.

umatrix-user-rules-xee5ch-custom-starter-template.txt

https-strict: * true
matrix-off: about-scheme true
matrix-off: behind-the-scene true
matrix-off: chrome-extension-scheme true
matrix-off: chrome-scheme true
matrix-off: localhost true
matrix-off: opera-scheme true
referrer-spoof: * true
ua-spoof: * true
* * * block
* * css allow
* * frame block
* * image allow