How to Transfer All Greasemonkey userscripts to Tampermonkey on Firefox 57+

How to export scripts from Greasemonkey to a zip file:

1. Click the Greasemonkey plugin
2. Click Export a backup...
3. Save file

How to import scripts to Tampermonkey from a zip file:

1. Click the Tampermonkey plugin
2. Click Dashboard
3. Navigate to Utilities tab
4. Click Browse... under Zip menu
5. Select the exported zip file
6. Click Import

Based on derjanb comment I'm answering the question showing the steps I took to achieve the transfer of the userscripts.

1. Enter about:support on firefox address, then find and click open folder

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2. Now find and open gm_scripts folder, click to search inside it and type .user.js, when the search finishes select all userscripts you want to transfer.

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3. Now right click the selected files and choose Send To - Compressed (zipped) folder.

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4. Once your zip file is created find it on Desktop or where it was created and open the Tampermonkey Dashboard

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5. Go to utilities tab and on Zip select import, find your zip file and once you click open button, it should load

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5. Finally a list with all scripts inside the zip should open, asking which one you desire to import. If you want all, just click import.

That should do it. Check it out on Installed userscripts tab.