YouRead от Demetrius

YouRead helps you make web pages easier to read by applying your preferred typography settings on a per-site basis.

Features:

Enable or disable YouRead for each website independently
Set a target CSS selector for the content area (for example: #main, article, .content)
Adjust maximum text width (in ch units) for comfortable line length
Adjust font size
Choose font family (Serif, Sans-Serif, Monospace)
Settings are saved per domain and synced via Firefox storage
How it works:

Open the YouRead popup on any site
Configure your settings for that site
Changes are applied to matching page content automatically