It's still on the classic File menu. If you don't display the menu bar full time, you can call it up temporarily by tapping the Alt key or the F10 key.

Thank you so much! How can I have the menu bar showing full time?

You may have found this by now, but if not, you can turn the Menu Bar, Bookmarks Toolbar, and other optional toolbars on and off using any of these methods:

• "3-bar" menu button > Customize > "Show/Hide Toolbars"
• tap the Alt key > View > Toolbars
• right-click a blank area of the tab bar
• right-click the "+" button on the tab bar

You can also find the Work Offline menu item in the Developer tools drop-down list. You can find the Developer tools button in the "3-bar" Firefox menu button list and possible drag this button on a toolbar.