Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
I strongly agree! I was thinking about that because I work on many different projects so tags are not enough.
I believe Xnote allows adding notes but the feature is not supported natively and is more likely to be discontinued. Ideally, such metadata could be synced across devices eg via x-headers on the server or through a database synced via Firefox Sync when it becomes available.