For the inactive tabs, is there a blank area where the title should appear, or do they look like "Application tabs" (pinned tabs) where only the icon of the site appears?

There is a blank area where the title should appear.

In case this is a glitch with the color scheme, e.g., black-on-black, could you hover the mouse over the tab to see whether the title is there but is not readable in the standard color scheme?

If that is the problem, and hovering gives you enough visibility, I have a custom style rule you can use to create that appearance on all the background tabs: http://userstyles.org/styles/101036/brighter-background-tabs-australis-firefox-29

If that's still too hard to read, a volunteer could help with a personalized style rule. In that case it would be helpful to see a screen shot of your tab bar. Please show a non-confidential/non-personal page. This article has tips: How do I create a screenshot of my problem?

Hovering didn't make a difference, so I ended up using the Classic Theme Restorer to fix the problem. Thanks for the help.