Updated.
On youtube, it doesn't work on video links, but works on non-youtube links in video descriptions.
Some hours later, it started working on duckduckgo, but I can't say what happened for the behavior to change
I'm also changing my rating from 4 to 5 stars. It's a shame it can't work 100% on youtube, but similar extensions in various browsers seem to have the same limitation with youtube and/or Single Page Apps in general.

Upgraded my review from a 1 to a 5, as it works just as it should. Thank you

this add-on works perfectly for me.

thanks to the developer.

firefox 120.0 64bit

Still going well in Ff 119 👍
What a pity that it doesn't work with Java 'Show or Hisìde'

browser.tabs.insertAfterCurrent is now workings correctly!

Works exactly as it should after setting the correct option. Must go to FF's 'Manage Your Extensions' page > Click the three dots [...] next to this addon > Select 'Options' (or the 'Options' tab on the 'Manage page) and select "Open a link to a different site on a tab" or "Open all links with tabs" depending on your preferences.