With sync, you can select the type of data you want to share. But, it's all or nothing as to that data.

That's why I ask the question because i don't have to be the only one wanting to share with my colleague only the professionnal bookmarks/password and not the personnal one :-)

Sorry, I don't understand your PC1, PC2 and your Bookmarks A, B, C, D, E, F example. Either Bookmarks in total are selected to be synchronized OR they are not, there is no way to have "different groupings" of bookmarks for the purpose of synchronization.

Overall, Firefox Sync shouldn't be used for that purpose. Beyond the types of data selected there is no user control over what is synchronized or in what direction the synchronization is done. All the selected Firefox data is sent to the Sync server which determines the data that needs to be sent to each device for "synchronization".

One solution is to Export your bookmarks from Firefox, and then edit out the bookmarks that you don't want to share with your colleague. Then email that bookmarks.html file to your colleague, who can then import that html file into his web browser. Virtually all web browsers have the ability to import a bookmarks.html file to merge with their bookmarks / "favourites".

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

When editing an html file the header must remain intact so that the 'receiving' web browser knows what that html file is used for.

This is the "header":

<!DOCTYPE NETSCAPE-Bookmark-file-1>
<!-- This is an automatically generated file.
     It will be read and overwritten.
     DO NOT EDIT! -->
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8">
<TITLE>Bookmarks</TITLE>

You will have to use separate Sync accounts with a different email address like you already are doing, but you will have tho use separate profiles where each profile is connected to a specific Sync account if you want to keep specific personal data separated like in your case some bookmarks. When you connect a profile to a specific Firefox Account then all bookmarks present in this account will be Synced. Like posted above, Sync is all or nothing and you can't select which bookmarks you want to include and which shouldn't be included.

I'll didn't know this notion of profile. I'll have a look at it, even if it seem's to be only a part of the solution.

Thank you.