IRSG/Let’s Encrypt filled the inclusion request in Mozilla trust store the 14 September 2015 : https://bugzilla.mozilla.org/show_bug.cgi?id=1204656
According to https://tsdr.uspto.gov/#caseNumber=86790719&caseType=SERIAL_NO&searchType=statusSearch Comodo CA filled a application for the trademark “Let’s Encrypt” the 20 October 2015, after the public inclusion request from IRSG, and long after the first public use of that mark by them.
It look like an attempt from Comodo CA to get the ability to impersonate another CA. Browsers should take actions to prevent that. A CA with such practice have no place in a trust store.
Update 25 June: It seems Comodo took the way to abandon that trademark. It’s a positive move to restore trust between CA.