Unless you want to add in your firefox login info to mull best option would be adding each bookmark to a spreadsheet and ~ manually adding each bookmark to your new browser ~
Edit: got a case of 2am brain over here and forgot an easier way. After saving the spreadsheet make sure its formatted correctly as hyperlinks that way you can export the spreadsheet as an HTML file. Depending on what desktop browser you can then import the HTML file in your bookmark manager
Yeah you can select multiple bookmarks on mull and "copy to clipboard" then paste them into whatever spreadsheet editor you want if that's what you meant? You can then format the links correctly on something like Calc or excel. I never personally tried extensions but that might be another route you can take. Sorry for all of the downvotes the privacy community isn't always this hostile to a simple question. Hope you have a good one