1) I have a bunch of older extensions, some of which aren't signed.
Extensions listed on AMO before signing was instituted were automatically signed. For extensions distributed off-AMO, fork them pursuant to their free software licenses and submit your fork as an unlisted extension.
2) Sometimes I modify extensions (fix bugs, get rid of annoying behaviors).
Then you are a developer, not a "consumer" who only views works created by others. Fork the older extensions pursuant to their free software licenses and submit your fork as an unlisted extension, or use Firefox Developer Edition.
3) I don't want to have to "apply" to someone else for the right to run something on my own computer.
Then use Firefox Developer Edition.