There are two Boolean (true/false) prefs in about:config to control what to show when you hover a tab. This pref controls whether to enable this new hover feature or only show the legacy tooltip with the title.

• browser.tabs.hoverPreview.enabled

This pref controls whether to show a thumbnail or a two line tooltip with title and URL.

• browser.tabs.hoverPreview.showThumbnails

The latter can be controlled via "Settings -> General -> Tabs".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Only a single tab shows up now for me. I cannot see all my open tabs to select through them quickly from the task bar. I have both options set to "TRUE" from above.