Set up your tabs as you wish. Then, make that your startup page.

https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

In the Bookmarks Manager, you can also set up a folder with those same links.

Right-click the folder and select Open All In Tabs.

Dick M said

If I mistakenly close the wrong window first I lose all my default session tabs and their pinned configuration.

Can you use one of these:

• "Library" toolbar button > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• "3-bar" menu button > Library > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• (menu bar) History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)

Is there a way to create a "default session" with pinned tabs that cannot be accidentally deleted?

Among built-in features, the "Open All in Tabs" in FredMcD's reply is the most practical approach for re-opening them, but you'll need to pin them yourself.

There also are session management extensions you could consider. These are the ones I'm aware of that you might investigate/try out:

Use database storage:

• https://addons.mozilla.org/firefox/addon/tab-session-manager/
• https://addons.mozilla.org/firefox/addon/session-boss/

Use bookmark storage:

• https://addons.mozilla.org/firefox/addon/session-sync/
• https://addons.mozilla.org/firefox/addon/my-sessions/

See also:

• StartupApps https://addons.mozilla.org/firefox/addon/startupapps/

Thank you all for your suggestions. I have already tried the various suggestions. Most lead to 5-10 minutes of re-configuration. I guess I'm spoiled. It seems to me that a Firefox feature which includes the specification of a default session consisting of a list of pinned tabs would help a lot of us.

In the meanwhile, I'm going to try cor-el's suggested addon "StartupApps".