doc: add doc for the Firefox backend

doc/bookmarkfs.texi

@@ -806,6 +806,72 @@ you may submit a feature request or implement one using the Backend API.
 @node Firefox
 @section Firefox Backend
 
+The Firefox backend provides access to the bookmark data of the web browser
+@uref{https://www.mozilla.org/en-US/firefox/new/, Mozilla Firefox}
+and its derivatives, notably @uref{https://www.torproject.org/, Tor Browser}
+and @uref{https://librewolf.net/, Librewolf}.
+
+Firefox bookmarks are stored in a SQLite database under the profile directory:
+
+@example
+~/.mozilla/firefox/@var{$@{profile_name@}}/places.sqlite
+@end example
+
+When mounting the filesystem, this pathname shall be passed as the @var{src}
+argument (@pxref{mount.bookmarkfs}).
+Actual path for the profile directories may differ across distributions.
+
+Backend-specific options:
+
+@table @option
+@item filename=title|guid
+Whether to use the bookmark title or GUID as the bookmark file name.
+Defaults to ``title''.
+
+A bookmark GUID is a base64url-encoded 128-bit string uniquely associated
+with a bookmark or bookmark folder.
+
+When creating a new file:
+
+@table @option
+@item @option{filename=title}
+The GUID is randomly generated by the backend.
+
+@item @option{filename=guid}
+The filename must be a valid GUID, and must not duplicate with other files
+on the same filesystem, otherwise @code{open()} fails with @code{EPERM}.
+
+Also sets the GUID string as the bookmark title.
+@end table
+
+@item lock=exclusive|normal
+The database connection locking mode for the bookmark storage.
+Defaults to ``normal'' when the filesystem is mounted read-only,
+``exclusive'' otherwise.
+
+This option corresponds to the
+@uref{https://www.sqlite.org/pragma.html#pragma_locking_mode,
+@code{locking_mode}} pragma on SQLite.
+With @option{lock=exclusive}, other process cannot access the bookmark storage
+until the filesystem is dismounted.
+
+The Firefox browser holds an exclusive lock on the bookmark storage by default.
+If you wish to mount the bookmarks while keeping the browser session open,
+turn off the @code{storage.sqlite.exclusiveLock.enabled} switch
+in @indicateurl{about:config}.
+
+@item assume_title_distinct
+If this options is provided, the backend assumes that bookmark names are
+distinct under the same bookmark folder.
+This option is ignored with @option{filename=guid}.
+
+This option may improve @code{readdir()} performance, however,
+making a false assumption results in a directory entry with duplicate names.
+It is recommended to perform a full filesystem check (@pxref{Filesystem Check})
+on the bookmark storage before mounting with this option.
+
+@end table
+
 
 @node Chromium
 @section Chromium Backend