DRM always is proprietary software. Disabling JavaScript, sending an empty
user agent, ... would make fingerprinting far easier, i.e., would work
against the "anonymity" goal. "Safe Browsing" does not "block non-free
content". It protects against malware, such as phishing thank to information
from... Google:
https://support.mozilla.org/en-US/kb/how-does-phishing-and-malware-protection-work
For anonymity, the Tor Browser is, today, the best. It is clear from what you
write that you would not do better. To not execute proprietary JavaScript, an
alternative to disabling JavaScript is the LibreJS extension. It is enabled
by default in GNU IceCat, which also has its own 100% free add-on site.
IceCat's default security and privacy are better than Firefox's too
(HTTPS-Everywhere, SpyBlock, some fingerprinting countermeasures).
As for GCC, it can compile itself but other C++ compilers can compile it as
well. That conceptually only pushes the problem further. However, the very
first compiler for a language can be written in another language (including
assembler) and there is the bootstrapping technique, which is very popular:
https://en.wikipedia.org/wiki/Bootstrapping_(compilers)
Anyway, I do not understanding why compiling with GCC would be a relevant
requirement (e.g., LLVM is free software too). Anyway, Firefox usually is
compiled with GCC.
