Bookmark ids

Add-ons Development
#1

Hello,
was wondering if bookmark ids always have 12 characters?
And also which characters are used: a-z, A-Z, and?
I will be grateful if you can send me this information.
Jan Zavřel

NOTE: already sent this question at dev-addons at mozilla.org but got this:
This email seems to be from a ProtonMail address but came from outside our system and failed our authentication requirements. It may be spoofed or improperly forwarded!

Is there something I can do to resolve this? My mail is jzav at protonmail.com and I am developer of this addon:
https://addons.mozilla.org/en-US/firefox/addon/at-your-command/

#2

It seems that my mail got through after all. Here is the answer I received - for future reference (many thanks to Jerry Krinock):

It’s a base 64 alphabet. In addition to the 52 Roman characters, it includes the ten decimal digits, and two punctuation characters underscore and hyphen. The thinking that went into the current design and patches to source code are in here:

https://bugzilla.mozilla.org/show_bug.cgi?id=607115

#3

I am still interested in this “spoofed or improperly forwarded” issue though. Is there something I can do to solve this?