png is larger in many cases.
It heavily depends on the type of picture you have. JPEG is mostly better for photos while PNG is better for pictures with much higher contrast, such as most webpages that do not include photos as their main contents.
I just tested with this thread's page as it was displayed on my computer before I posted, and got a 449 KB .PNG file on one side, and a 1.2 MB .jpg file on the other side.
Trying to save to JPEG that same picture (starting from the lighter, lossless version) with enough compression so that it goes below 450 KB (using IrfanView, either with the RIOT plug-in and specifying a size, or without the RIOT plug-in but by trying various possible values for the quality slider) produced pictures with lots of artefacts around the text, which is really ugly.
Also, the PNG file I obtained can be quickly optimized to become even smaller, still without quality loss, with programs such as PngOptimizer
: I got it down to 372 KB.
Pearl Crescent Page Save is Firefox 57 (Quantum) and I use lower Firefoxes like 52.
Only the version called Page Saver WE (for WebExtensions) requires 57+, while this one
is for earlier versions.