You're probably seeing the new Multi-process feature which Firefox is gradually moving towards.

https://blog.mozilla.org/futurereleases/2016/08/02/whats-next-for-multi-process-firefox/

Multi-process Firefox is codenamed "e10s". https://wiki.mozilla.org/Firefox/multiprocess

Started with Firefox 48 for a very small percentage of users who had no extensions installed; expanded in Firefox 49 to a slightly larger group of users; further expansion in Firefox 50. Automatically disabled when too many extensions installed are detected and when extensions that don't yet support e10s are detected.

Extension compatibility for the most popular extensions which are hosted at AMO are listed here: http://arewee10syet.com/

To disable multiprocess:

Type about:config into the address bar, press Enter, accept the warning. Out this in the Search field at the top browser.tabs.remote.autostart . Double-click that preference in the reduced listing that is below the Search field to toggle the 'Value' from True to False.

Multiprocess should make Firefox more stable and allow you to use the interface even if individual tabs are loading slowly. Yes, it uses more memory, but I wouldn't think it uses a lot more.

To address high memory use more generally, please check out the following article:

Firefox uses too much memory or CPU resources - How to fix