Hi Chuck,
thank you for your quick response. As I told, FEBE does perform the backup normally (though a bit slow), but it freezes when I click "Check AddonManager functionality" within FEBE's "Tools" panel.
Here is what I did:
-- Clear FEBE preferences
-- Restart Waterfox
-- Open error Console
-- click FEBE/Perform backup ==> backup runs as normal (but a bit slower than it used to be with Firefox on the same laptop PC); no error message
-- open FEBE Options/Advanced/Tools
-- click "Set read/write permissions" => no error
-- click "Attempt to load and save extension data" ==> no error
-- click "Verify extension directory" ==> no error
-- click "List installed add-ons" ==> no error
-- click "Check AddonManager functionality" ==> blank window opens and FEBE/Waterfox seems to freeze; after 2 minutes or so the following message pops up:

- Script not answering
-- click "Stop script" ==> The window shows detailed information about the addons. Unfortunately I was unable to copy the contents of that window, but it was a rather long list including informations about many language packs which seem to be integrated within Waterfox.
-- click on "Debug script" ==> the error console shows this:

To me it seems that this is a "Waterfox-specific" problem resulting from the fact that this browser obviously contains dozens of language packs (unlike Firefox where I use language specific installation files) that are treated by FEBE as normal "addons", thus soon being "overloaded" by the big amount of addon data. Eventually it would be reasonable if FEBE users could decide what kind of addons (i.e. "real" extensions, plugins, or language packs) FEBE should look for when they click on "Check AddonManager functionality".