hello minkowski, you can create a dedicated bookmarks folder for those tabs - right click the tab-bar & select bookmark all tabs...
afterwards you can open them all at once by middle-clicking or holding the ctrl-key while clicking the folder.

The problem is that my Pinned Tabs tend to disappear. With a bookmark for Multiple Tabs I can quickly recreate the lost Pinned Tabs How can I avoid losing the Pinned Tabs ?

In case you are using "Clear history when Firefox closes":

• do not clear the Cache (Firefox will disable the disk cache in this case, see about:cache)
• do not clear the Browsing History
• do not clear the Cookies
• do not clear the Site Preferences

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data.

Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox profile folder.

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Multiple_profile_files_created

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).