Just a theme, no, I don't believe so. A full-fledged extension, possibly, though I'm sure hat would be a breach of the Mozilla Add-on policy.

From Add-ons for Firefox > Frequently Asked Questions:

Are add-ons safe to install?
Unless clearly marked otherwise, add-ons available from this gallery have been checked and approved by Mozilla's team of editors and are safe to install. We recommend that you only install approved add-ons. If you wish to install unapproved add-ons or add-ons from third-party websites, use caution as these add-ons may harm your computer or violate your privacy. Learn more about our approval process

What does it mean if an add-on is "experimental" or "preliminarily reviewed"?
Experimental add-ons have been checked by our editors to make sure they don't have security problems, but they may still have bugs or not work properly. Use caution when installing experimental add-ons and uninstall the add-on immediately if you notice problems. Learn more about our review process

What does it mean if an add-on is "not reviewed"?
While all add-ons publicly available in our gallery are reviewed by an editor, you may receive a direct link to an add-on that hasn't yet been reviewed. Use caution when installing these add-ons, as they could harm your computer or violate your privacy. We recommend that you only install reviewed add-ons. Learn more about our review process

Tylerdowner wrote:

Just a theme, no, I don't believe so.

It's incorrect to suggest that a theme doesn't have the same technical capabilities. Go look at the plethora of themes that are bundled with an extension, e.g. Silvermel, Charamel, Oxygen KDE, etc.

Gingerbread_Man, as i said "Just a theme...". A pure theme mainly changes CSS and images (in a nutshell). Bundled with extensions, or other cases where it isn't purely a theme, was covered by my qualifier "A full-fledged extension, possibly".

The answer is, imcurious1, that the reviewed and approved extensions and themes on Mozilla's add-on website are safe, Un-reviewed add-ons from elsewhere may not be.

I did read the answer. What did I miss? Are you suggesting that

• To most users, it's obvious that a theme like Silvermel which appears in the Themes category isn't “just a theme”?
• That most people pay attention to the target URL of the Add to Firefox button and are able to differentiate *.jar from *.xpi?

I just answered the question. No, not all users will be able to tell the difference, but some will. Those that can, good, they know the difference. Those that can't can see that add-ons posted on AMO are reviewed and safe.

(this is drifting away from the topic of the thread and not helping anybody so let's stop now)