This feature is very disruptive. In the modern web world many of the pages are bulky and slow to load. I propose a few modifications to this feature as I believe it is a good feature but needs some changes.
Increase the default time to "clear" a page and remove it from memory. It feels like it clears it off after such a short while.
Enable the feature to be toggled on or off complete
Enable the feature settings to be "fine tuned" to suit the needs of the user. I would probably set it to ~2hrs based on my usage, as I jump back to firefox after working on features and now it is a big disruption to have to reload most pages that I am switching between...I have been a long time loyal firefox user but this is making me want to use chrome or something else...