A theme is an add-on that changes the visual appearance of Firefox. When several themes are installed, you can switch to another theme as follows:

1. Click the menu button and choose Add-ons.
2. Switch to the Appearance tab. You should find your previously used personas in the list.
3. Find the one you want to use and click the Enable button.

To uninstall the other ones, you can click the Remove button on them and they'll be uninstalled instantly.

For more information about Firefox themes, see the article Use themes to change the look of Firefox.

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Wesley Branton said

A theme is an add-on that changes the visual appearance of Firefox. When several themes are installed, you can switch to another theme as follows:

1. Click the menu button and choose Add-ons.
2. Switch to the Appearance tab. You should find your previously used personas in the list.
3. Find the one you want to use and click the Enable button.

To uninstall the other ones, you can click the Remove button on them and they'll be uninstalled instantly. For more information about Firefox themes, see the article Use themes to change the look of Firefox. Did this answer your question? Let us know.

I would know if I added a theme between versions. I didn't touch themes, the default theme went from transparent in ver 42 to solid in 43. Are you telling me you are not seeing this on your side?

Unfortunately, I don't have a Mac device to test this issue on.

It is possible that the developers made a bit of a mistake with the latest version. Please file a bug report on Bugzilla and our developers will have a look into the issue.

Also, please note that a patch for Firefox (Firefox 43.0.1) was released today. Please try updating your Firefox, as this could patch the issue.

Sorry for any inconvenience.

You can check that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; never remember history) in case that makes a difference.

• https://support.mozilla.org/kb/Private+Browsing

Nobody else has this problem or has noticed it???? Version 42 was transparent and looked great, 43 ruined it and it still isn't resolved in 43.0.3.

No, I am not using a theme and no I am not in private browsing mode. I am not dumb, I did nothing different, the transparent theme of the window is just gone.

Per help on Reddit:

This bug removed vibrancy (blur effect) in 44: https://bugzilla.mozilla.org/show_bug.cgi?id=1161565 The patch was then uplifted to 43. You should be able to bring back vibrancy using an user style:

1. titlebar {

 -moz-appearance: -moz-mac-vibrancy-light

} You can also get dark vibrancy by using -moz-mac-vibrancy-dark

Here is a similar workaround: http://apps-reviewed.blogspot.com/2015/01/make-firefox-translucent-in-os-x.html