Your assumption is incorrect, Firefox addons are not inherently more secure than Chrome extensions (though in terms of security, I attach more value to Firefox's official addon gallery (AMO) than the Chrome Web Store because all addons on AMO are manually reviewed).
Addons in Firefox are trusted by design; they can do anything that is allowed by the Firefox process.
The worst-case scenario for installing a malicious Firefox addon is a re-install of your operating system to clean up the mess.
The Chrome extension APIs are very constrained, because the Chrome browser does not fully trust extensions (unlike Firefox). Chromium extensions can typically not access any resources outside Chrome's sandbox without the user's approval. The worst-case scenario in Chrome is less severe than Firefox' (and also applicable to Firefox): All of your web browsing activities can be considered compromised.
Since most of us increasingly spend more time in the web browser than in native applications (e.g. internet banking, e-mail), it is a stupid act to install extensions that you cannot trust. Considering this view, then Chrome extensions could be more secure because you have to consent to the every newly requested permissions upon installation/update. Firefox has not implemented any addon permission warnings, so when you install a Firefox addon, you should always mentally add a "This addon could access all data on your computer and the websites you visit" warning to the installation dialog.
On the other hand, because Firefox addons are more powerful, they can also integrate security features in a much better way than Chrome extensions. For example, to date it is not possible to create a NoScript equivalent in Chrome because of the limited extension API.
If you want to know more about Chrome's extension security design, I suggest to read: