What is the exact address of your bookmark? To check on that, you can right-click the bookmark, choose Properties, then look at the Location line.

Does this address work any better:

https://mail.google.com/mail/

Your Firefox identified itself as version 61. Is that correct? You can verify your current version on the Troubleshooting Information page. 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, you should find a row with the version number.

If it's 60.something in the esr series or 62.something, your user agent may need to be reset. See: How to reset the default user agent on Firefox.

If it's not one of those versions, see: Update Firefox to the latest release.

There are other things that need attention.

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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