docs(browser support): added a "behind a flag" implementation for Firefox (#4684)

Author: mfranzke

packages/foundations/docs/BrowserSupport.md

@@ -23,7 +23,7 @@ The following functionality relies on JavaScript for support in browsers that do
 | [`@property` / typed CSS Custom Properties](https://developer.mozilla.org/en-US/docs/Web/CSS/@property)                                                                                  | 85 (25.08.2022)  | 128 (09.07.2024) | 16.4 (27.03.2023)                                                                                                    |
 | [`light-dark()` CSS function](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/light-dark)                                                                                   | 123 (19.03.2024) | 120 (21.11.2023) | 17.5 (13.05.2024)                                                                                                    |
 | [`attr()` CSS Values 5 extensions](https://developer.mozilla.org/en-US/docs/Web/CSS/attr) (planned)                                                                                      | 133 (04.02.2025) | tbd              | tbd                                                                                                                  |
-| [`style()` container queries for custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_containment/Container_size_and_style_queries#container_style_queries) (planned) | 111 (07.03.2023) | tbd              | 18 (16.09.2024)                                                                                                      |
+| [`style()` container queries for custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_containment/Container_size_and_style_queries#container_style_queries) (planned) | 111 (07.03.2023) | _behind a flag_  | 18 (16.09.2024)                                                                                                      |
 | [Anchor Positioning](https://caniuse.com/css-anchor-positioning) (planned)                                                                                                               | 125 (14.05.2024) | tbd              | ["this fall"](https://webkit.org/blog/16993/news-from-wwdc25-web-technology-coming-this-fall-in-safari-26-beta/#css) |
 | [CSS Custom Functions](https://www.bram.us/2025/02/09/css-custom-functions-teaser/) (planned)                                                                                            | 139 (30.07.2025) | tbd              | tbd                                                                                                                  |
 | [Invoker Commands](https://caniuse.com/mdn-html_elements_button_commandfor) (planned)                                                                                                    | 135 (01.04.2025) | Nightly          | Technology Preview (211)                                                                                             |