Ctrl+d bookmarks the current tab. Not sure if there are bookmarks for selected tabs but you could see what happens if you hit that while you have more than one tab selected.
Firefox
/c/firefox
A place to discuss the news and latest developments on the open-source browser Firefox.
Rules
1. Adhere to the instance rules
2. Be kind to one another
3. Communicate in a civil manner
Reporting
If you would like to bring an issue to the moderators attention, please use the "Create Report" feature on the offending comment or post and it will be reviewed as time allows.
It does in fact only bookmark the first tab when multiple are selected.
After looking over some of your posts on these Firefox communities, I believe you're well into "time to make my own addon" territory.
I've written a single addon myself, and while I found documentation to be lacking, it isn't as hard as it first feels. What really helped is finding other plugins on GitHub that did something somewhat related to what I was attempting, which gives you a solid base.
Oh you're absolutely right, I've been in that territory for awhile ago, I haven't gotten time until lately to make addons.