@ertott this is an interesting case, for sure.
May I ask you to edit one line of code to see if it fixes things? If you can, I need you to deactivate SBP and then open the following file:
/wp-content/plugins/speed-booster-pack/includes/classes/class-sbp-lazy-loader.php
And go to line 37:
wp_enqueue_script( 'sbp-lazy-load', SBP_URL . 'public/js/lazyload.js', false, '17.7.0' );
And change the line with the following:
wp_enqueue_script( 'sbp-lazy-load', SBP_URL . 'public/js/lazyload.js', false, '17.7.0', true );
After saving the file, clear the cache and try again, see if Firefox handles lazy-loading better this time.
Alternatively, you can try disabling every SBP option except for lazy loading and see if it’s fixed. If it helps, then we can deduce that another feature of SBP is conflicting with the lazy loading feature.