I think the main problem here is the ideology of software updating. Updates represent a tradeoff: On one hand there might be security vulnerabilities that need an update to fix, and developers don't want to receive bug reports or maintain server infrastructure for obsolete versions. On the other hand, the developer might make decisions users don't want, or turn even temporarily (as in a supply chain attack) or perman…
It's less than ideal but you can 1) load extensions temporarily in about:debugging, 2) turn off xpinstall.signatures.required in nightly or dev edition to install them for good or 3) sign on addons.mozilla.org without publishing to the marketplace.