Google clearly shows their intent by not providing an alternative API for content filtering, but that doesn’t mean there are no security concerns. Malicious extensions have become so prevalent that Mozilla had to switch to only permitting signed extensions (despite community outroar) because shitty companies were inserting their extensions into the users’ profile directory without permission and breaking websites and even Firefox itself in some cases.
Secure Boot requires the user to be able to turn it off, so if it gets in the way of anyone, it’s implemented wrong. Microsoft has a weird certification system for “super duper secure” laptops or whatever they call it where only their private key is loaded, but that’s a small amount of expensive business laptops.
If anything, Secure Boot is an example of the “just let me turn it off if I want to” crowd making computers less secure for the majority because Microsoft allows booting a whole bunch of Linux distros on supposedly locked-down systems, which has been proven to make other attacks possible (like that recent one on Lenovo laptops where a Linux boot disk could insert a fingerprint into the fingerprint reader that would unlock TPM-based encryption).
Nobody is preventing you from installing Linux through secure boot. In fact, you can take control of your secure boot settings and prevent anyone from installing Windows on your computer without your password.