When someone replies inline to one of my messages, their text is hardly distinguishable from mine. Is there a setting or a hack to improve this?
Yes, there is: Use user styles.
One alternative could be Gmail quote colors and font sizes. I didn't tried it yet.
How user styles are different than user scripts
User styles exist mainly to change the way things look. They are written with CSS, the same language that web sites use to control their appearance. Most browsers allow for user styles out of the box (with varying degrees of ease).
There exists a fair bit of overlap between user scripts and user styles. Some things can be accomplished with either method. User styles can change the user interface in Mozilla-based programs such as Firefox and Thunderbird, while user scripts cannot. User scripts can add behaviour to web sites, while user styles cannot. In general, it is recommended that if something can be done with user styles instead of user scripts, it should be done with user styles instead of user scripts. This is because user scripts can cause a site to flicker, as they have to wait for sites to load completely before firing, while user styles are not under this restriction. User styles are also generally easier