This is not possible without a browser extension. Luckily, most extensions used for this purpose have a minimal impact on your browser's performance.

The Shortkeys (Custom Keyboard Shortcuts) for Firefox extension should do the trick for you.

For the record, if you choose to use that extension (this is an issue with others as well), you can't override a builtin Firefox shortcut by setting the Ctrl + S action to Do nothing. Instead, set it to Run Javascript and then just leave the Javascript section blank.

Hope this helps.

Wesley Branton said

This is not possible without a browser extension. Luckily, most extensions used for this purpose have a minimal impact on your browser's performance. The Shortkeys (Custom Keyboard Shortcuts) for Firefox extension should do the trick for you. For the record, if you choose to use that extension (this is an issue with others as well), you can't override a builtin Firefox shortcut by setting the Ctrl + S action to Do nothing. Instead, set it to Run Javascript and then just leave the Javascript section blank. Hope this helps.

Will it cause more lag to the browser? Everything counts here. I've followed the procedures for boosting the browsers performance but nothing seems happening.

Theoretically, every extension comes with a small performance decrease. In reality though, the performance decrease caused by an extension of this nature would be so negligible that it would have no detectable impact on your browser's performance.

Sometimes the extension doesn't work and when I open the browser, I can still do the shortcut.

Nope, disregard what I said. It only worked the first time but after that, it stopped working.