Open the extension's options either by opening the list of extensions, clicking the cog icon and then clicking "manage extension". Then select the options tab.
You can also get there through Settings > Extensions & Themes, then clicking the add-on and selecting the options tab.
In this tab you can use the export history button to download a data.json file containing all of your history. The other button can be used in another firefox instance to select a file to import history from.
