thanks for your reply
i checked again and found two things:

1.
some working links were only reported as broken when the tracking protection in Firefox (disconnect.me / basic protection) is on; i know it was stated to set it of, but i thought since v.2.x the add-on would take care of that, sorry my fault
(just for info, these were some of the links: http://statcounter.com/ ; https://www.forbes.com/ ; http://www.ip-tracker.org/ ; https://twitter.com/ ; ...)

2.
still, with tracking protection off, there are some other working links reported as broken; it seems to occur when a site redirects to a localized version
(e.g.: bookmark = https://www.seagate.com reported as broken, but it opens https://www.seagate.com/be/nl/ ; also bookmark = https://www.microsoft.com/nl-be/default.aspx reported as broken, but it opens the same link) ; (note: a second run gave the same result)

hope this helps, thanks again for this very usefull extension, 5 stars now

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about a month ago i had problems with the add-on hanging at the last bookmark check(s); all fixed now in version 2.x, works like a charm (also fast: 1600+ bookmarks scanned in a few minutes)

4 STARS ! (only reason for that one star less is that some working links are reported as broken (e.g. twitter) but i understand some false positives are nearly inevitable 'cause site settings, etc.)

big thanks to the developer