You can select a theme that has dark text on the tabs (toolbars) if you currently use a theme with light text.

You can change this pref on the about:config page from 3 to 2 to revert to the Firefox 94 behavior regarding to prefer dark or light background for webpages.

• layout.css.prefers-color-scheme.content-override [dark:0, light:1, system:2, browser:3]

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Just to illustrate how Firefox does theming now, you can compare these examples:

Dark text on toolbars => dark text on light background

Light text on toolbars => light text on dark background

Individual websites may vary, though, so if the problem with with a specific site not following Firefox's general theming, it would be helpful to know more about that site.