I don't see anything to do what your asking. I would surmiss that would be something your router would do the Browser doesn't control bandwidth that comes from the ISP.

Hello marmot2,

With this add-on you can set the resolution to whichever you want, but on YouTube only :

https://addons.mozilla.org/en-US/firefox/addon/youtube-high-definition/

Then I found this add-on :

https://addons.mozilla.org/en-US/firefox/addon/save-data/?src=search

Worth a try .....

Note that the OP wants the lowest resolution and not the highest to reduce the data transfer as much as possible.

Quote: Data costs a lot of money

For videos on YouTube this might be possible if YouTube remembers your choice. You may have to login to keep this setting or at least make sure to keep YouTube (Google) cookies.

• https://support.google.com/youtube/answer/91449

For images best would be to block images and only load the images you want to see. There is usually no way to select a lower quality for images unless you would use a proxy or special service that re-encodes the images and send lower quality. There are extensions that can block images, but not of all images the file size is known. You can search the Addons website.

cor-el said

Quote: Data costs a lot of money

That's why I suggested this add-on, as under "About this extension" it says :

"Tell websites that you prefer to save data and costs."

https://addons.mozilla.org/en-US/firefox/addon/save-data/?src=search