Reading Ruler is a free Firefox extension designed to assist users in focusing on text while reading online. It highlights the line of text directly under the mouse cursor, making it particularly useful for articles, blogs, and lengthy posts. Users can easily toggle the visibility of the ruler using a hotkey or through the options menu, allowing for a customizable reading experience tailored to individual preferences. Additionally, the ruler's color and opacity can be adjusted for optimal contrast, further enhancing readability.

This open-source tool emphasizes user privacy, as it does not collect data, send information to the cloud, or display advertisements. Reading Ruler is a straightforward solution for anyone looking to improve their reading efficiency on the web, providing a distraction-free environment that allows users to maintain their focus and return to their reading seamlessly.