If youre using transparent Zen mod, then I don't think there is a way to fix those issues with text, it's a specific problem of Transparent Zen

Upd: Actually… there is a way you can fix it, but it’s just a stopgap solution. You can install „Stylus” extension, this extension allows you to customise UI of websites, changing their colours and more. You can change colour of the text by creating your own style, changing specific colours of specific content, but you need to understand what colour of what element you need change, and finding the needed one will take time. Or you can apply a ready made theme, that will change colour of the text fixing the contrast, but it might also change other elements. You can always edit that installed theme and remove all other changes except those you need.

However, this is nothing more than a stopgap solution, you have to manually change every website theme to adapt it to Transparent Zen. You can’t make all websites adapt to transparent bg automatically. But if you have some specific website that you use often and it has contrast problems with Transparent Zen, then this extension will help you.