No CSS styling on Firefox and IE with Customizr (child) theme

Rule of thumb: CSS always works as expected. If some rule does not apply in a browser but it applies in others, it is from one of these causes:

1. Your browser didn’t load the new (modified) stylesheet and uses a cached older copy of it. You need to force-refresh the page to fix this.
2. Your CSS has browser specific selectors (WP appends a browser-specific classes to the body element, allowing CSS fixes for specific browsers). Check your CSS for any of the following selectors (.ie, .gecko, .chrome, .safari, .opera).
3. Your CSS has browser specific CSS attributes (prefixed with -webkit, -moz, -o or -ie). If this is the case, you need to add the CSS that works in your browser.
4. You have errors in your CSS that prevent the browser to read your declaration properly (a common mistake is not closing curly brackets of a css rule, this making most browsers ignore the selector of the next CSS rule, since it thinks it is still inside the previous rule).
5. There is a stronger CSS selector that overrides your code but it only loads in your current browser (some themes load different stylesheets for different browsers – especially for IE).

Having said that, I should add that CSS debugging can be tricky and sometimes misleading, so your best bet is to provide a link and let us know what you are trying to achieve.