You can middle-click a bookmarks folder to open all the bookmarks in that folder or use the right-click context menu. You can select multiple bookmarks in the usual way via Shift and cursor Down and by holding down the Ctrl key and left click to select individual items. In the bookmarks sidebar you need to click at the left of the icon to prevent opening the bookmark if you want to select multiple items.

Maybe I wasn't clear, I was very tired when I posted this answer. My problem is not opening multiple bookmarks at the same time. My problem is to open multiple bookmarks in an intended order. If I just mark the bookmarks and open them they will open in an 'inverse order' (the first bookmark of the list will be at the furthermost right position in the tabs, etc. ). If I try to invert the order of the bookmarks in the bookmarks windows with the A>Z Z>A command the bookmarks will open in some random order (it seemed alphabetical to me yesterday, it's not. it's just random.)

The end result is that I can't really use the function unless I reorder all the (possibly many) tabs that will be opened by the function.

Try to toggle the browser.tabs.insertRelatedAfterCurrent pref on the about:config page to false.

I ended up finding a way myself, in the bookmarks window select "order by date added" then inverting the bookmarks order marking the "Z>A" option and finally just mark the bookmarks and opening them.