There's really no good forward if you're stuck in where the author is.
Either you upgrade and lose several important addons, and functionality in other addons. These are often what makes Firefox a good browser (for you).
The second option is to downgrade to ESR, which will delay the need to upgrade for a few more months. This may invalidate parts of your profile. It also means you lose out on a number of significant performance improvements included in 56.
And the last option, of course, the author's choice (which recommend against): staying on 56. This is of course a bad idea for the reason you stated, plus you lose out on future performance improvements and possibly new web technologies. You will also keep getting nagged by Firefox to upgrade.
So, there is no good option, and you have to pick your poison: Upgrade past 57 suffer significant amounts of usability/efficiency, downgrade to 52 and lose parts of your profile and get significantly worse performance, or stay on 56 and risk the browser getting compromised.
I hate to victim blame, but he's entirely at fault for the predicament he's in. It was widely known that legacy extensions would be phased out it 56, and a few searches related to firefox 56 will tell you that ESR would be the only branch that supports both legacy extensions AND will recieve security patches past 56. So he should have switched over to ESR before 53 came out.
Either you upgrade and lose several important addons, and functionality in other addons. These are often what makes Firefox a good browser (for you).
The second option is to downgrade to ESR, which will delay the need to upgrade for a few more months. This may invalidate parts of your profile. It also means you lose out on a number of significant performance improvements included in 56.
And the last option, of course, the author's choice (which recommend against): staying on 56. This is of course a bad idea for the reason you stated, plus you lose out on future performance improvements and possibly new web technologies. You will also keep getting nagged by Firefox to upgrade.
So, there is no good option, and you have to pick your poison: Upgrade past 57 suffer significant amounts of usability/efficiency, downgrade to 52 and lose parts of your profile and get significantly worse performance, or stay on 56 and risk the browser getting compromised.