Hi,
Since Personas Plus is no more compatible with Firefox 55+, is there an easy way to test lightweight themes, without having to submit them on AMO?
Thanks!
Hi @nico! Unfortunately, there is currently no other way to test lightweight themes. A developer is currently working on migrating Personas Plus to WebExtensions APIs, so hopefully a compatible version will be available in the upcoming months.
I have a suggestion but it could be a little time consuming when you compare it with how easy it was with Personas Plus. So if you want to give it a go, this is what I would recommend for now.
Go to the Themes area and search for a solid color theme, particularly an unusual color. For this example, I did a lime green search but then picked a light green color. Hover over the theme so that it displays on your browser header.
Take a print screen and open it in your graphics program. Now take your magic wand tool and select the theme color.
Delete the selected area. You will still have your address bar, search bar and other icons viewable.
Take your saved theme design and in a new layer BEHIND your print screen layer, drag and drop or copy and paste your design into the new layer and align it from the top right like the theme does on the site.
I realize viewing it this way isn’t the same thing as in your browser, but it might be helpful for whatever your reason for testing it is. I’d also recommend you save the print screen as an editable template for future use, if you decide to try it. 
Ok, so nothing really convenient for now. This is to answer a user on the French support forums, but I assume other people may have the same issue.
Based on the snippet that was available on a previous version of LWT page on MDN, I’ve developed in simple tool to be able to install a theme without uploading it on AMO: http://nicopensource.free.fr/s/personaext/. Feel free to use it!
Its main limitation is that you need to upload your image somewhere first. Live previewing also isn’t working, I am not sure why, since the “preview” event is fired similarly to the “install” event, but nothing happens and there is no error.