I agree with jholland1964 that likely some culprit is causing the issue

........having said that;

with Firefox open, select the "Bookmarks" from the menu and click on "Organize Bookmarks"
in the panel at the left of the Bookmarks Manager Window, highlight the "Bookmarks" ___ in this way you are selecting all of the bookmarks within the folder ___ Then click on "File" at the top left of the menu and choose "Export"
This will prompt you to save a file, "bookmarks.html" __ then navigate to someplace where you will place that file, such as the Desktop then click the "Save" button.

You should now see a file on the Desktop, or wherever you directed it to be created.

Now at this point I am admittedly a bit fuzzy as I have never used Slimbrowser
however, in that it in fact relies upon Internet Explorer in order to function at all, I will assume that the Import function is similar if not identical
Therefor, open Slimbrowser, select File from the uppermost menu and click on Import/Export
Follow the steps and indicate you will be importing from a file
when prompted, navigate to the bookmarks.html you created on your desktop
and select it for import

hope that helped