What do you mean by grayed out? Is it simply faded or is it completely gone?

They are faded so you can’t click them.

Do you have a Recently Closed Windows item on your History menu? Perhaps the missing tabs are in a different window.

Generally speaking, if "Restore Previous Session" is grayed or missing, then Firefox either

• already restored the session, or
• didn't find a session history file to restore

Did this just become a problem recently -- were you able to restore sessions before?

I used Firefox as my default browser about a year ago. At that time (several versions back) I was able to restore my session. I just started using Firefox as my default browser again and so I have now had to restore my session since then. I had to reboot my iMac and my session didn't restore. I started looking around and that's when I found these grayed-out.

Okay, I don't know what caused that. Here are some things to look at:

Back Up Session Data Files, if any

Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you start by making a backup of your existing session history files? Here's how:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show in Finder" button. That should launch Finder to your profile folder. If you don't see a folder named sessionstore-backups right away, open the folder that Finder has selected and look inside.

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update
• various .js files from Firefox 55 or earlier

If you check the date/time of the various files, do you think any of them would have the missing tabs?

To preview the contents of a file, you can drag and drop it onto this page, then click Scrounge URLs: https://www.jeffersonscher.com/ffu/scrounger.html

That tool is on my site, so please let me know if it doesn't work for you.

Settings to Minimize Session Loss

You can set Firefox to restore your session automatically at startup. That can be found here:

• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Windows: "3-bar" menu button (or Tools menu) > Options
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

At the top of the "General" panel is the Startup section with "Restore previous session" -- check that box.

Don't set Firefox to clear history when it closes

In the left column, click Privacy & Security

On the right side, in the History section, there is a main setting:

• "Firefox will: Temember history" -- all should be okay

• "Firefox will: Never remember history" -- Firefox won't be able to restore your session if this is selected

• "Firefox will: Use custom settings for history" -- With this setting, check:
  • "Always use private browsing mode" -- Firefox won't be able to restore your session if this is checked
  • "Clear history when Firefox closes" setting:
    • not checked, ok
    • checked => click the Settings button and make sure "Browsing & Download History" is NOT set to be cleared