In your Firefox profile there's an sqlite database called Places, which after a while starts resembling a swollen Swiss cheese with holes. That might slow your Firefox down.

You can speed your Firefox up a bit, by optimizing (vacuuming) that database: you can namely deflate that swollen Swiss cheese into a compact smaller cheese. As follows:

Type the following in the URL bar of Firefox:

about:support

Press Enter.

Almost at the bottom of the page you get to see then, there's a header called Places Database. Click there on the button called Verify Integrity.

You're done! Repeat this on a monthly basis, so that your Firefox won't lose speed again because of a swollen database.