That's only true on iOS. On the desktop and Android it does use Blink, see https://twitter.com/brendaneich/status/701019625006489600.

Unfortunately as is mentioned on their add-on page at https://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer/ Classic Theme Restorer will no longer work in Firefox 57 which is expected to be released in November 2017. Mozilla is dropping support for XUL/legacy add-ons.

My parents' computer was also suffering from the Windows Update checks consuming a lot of memory. Microsoft has released update 3050265 to fix this problem which did greatly reduce the memory usage during checks. Note that the update is only listed an optional update so you need to manually select that update for installation.