Hello!
Librewolf optionally uses an overwrite file, which modifies the config of the current profile at every launch.
On a Windows install, it is located at %USERPROFILE%\.librewolf\librewolf.overrides.cfg
(When using the "official" portable version from librewolf.net, it is located at Profiles\Default\librewolf.overrides.cfg )
See: documentation on librewolf.net
Unfortunately, the PortableApps version uses the original location:%USERPROFILE%\.librewolf\librewolf.overrides.cfg
(The folder and the file may have to be created manually)
Could you please modify your version to use ...\PortableApps\LibreWolfPortable\Data\profile\librewolf.overrides.cfg
For testing, create a text file named librewolf.overrides.cfg in the mentioned location.
It's content should be this line:pref("browser.toolbars.bookmarks.visibility", "never");
The above test file, when successfully loaded, will disable the bookmark toolbar at every program start.
You can also check it in about:config. The valid values for browser.toolbars.bookmarks.visibility are never,newtab andalways (default).