Love this add-on, but also for me the settings were not adjustable (the "save" button isn't visible in the embedded settings page).

I could solve this by:
1. click the sloth icon in the toolbar, right click on the panel, click "view page source".
2. Go to the bottom of the source code and find the link that looks like this: "Options".
3. Right click on the "#" and "inspect".
4. In the inspector pane copy the value of the "href" attribute (eg: "view-source:moz-extension://ddfs4343d-7c2a-fa49-9177-459324914ed0/html/popup.html#").
5. Now replace the "popup.html" part of the link with "options.html"
6. Paste the resulting link into your address bar, and remove the "view-source:" part. Adjust your settings and save :-)

NIce addon! The options page should open in a new tab. I could save my rate only by opening moz-extension://05160fd2-bec1-4335-be5c-2739f3610c15/html/options.html

Works well but can't save change of currency format. And it should only counts when the tab is active (or at least we should be able to set it so). And setting sub-domains could be a nice bonus (some parts of some sites are acceptables, some not).

Really makes me want to get the heck off YouTube

Same issue here: Currency doesn't stick after restart of the browser, even after recent update.

Update: With 1.4 it finally works. Thanks!