Fit Text does not scale correctly inside Media & Text block in Firefox

Description

When "Fit text" is enabled (Typography > Fit text toggle) on a Paragraph or Heading block placed inside a Media & Text block, the text does not scale correctly in Firefox. It renders correctly in Chrome, Edge, Brave, and Safari.

The text either overflows the content area or renders at a size that doesn't fit the container — on both frontend and backend (editor).

This appears to be the CSS Grid counterpart of the flex container issue fixed in #73652. That PR fixed fitText inside flex containers, but the Media & Text block uses a CSS Grid layout (grid-template-columns: 50% 1fr), and the same class of problem occurs here — the fitText algorithm miscalculates the available width inside the 1fr grid column in Firefox.

I also attempted the CSS-only fix of adding min-width: 0 to .wp-block-media-text__content (which is a standard fix for grid item intrinsic sizing in Firefox), but this did not resolve the issue. The problem appears to be in how the fitText JavaScript reads the container dimensions inside a CSS Grid context in Firefox, not just in how Firefox resolves min-width: auto.

Step-by-step reproduction instructions

1. Open Firefox (tested with v147.0.4 and Developer Edition v148.0b15)
2. Create a new post
3. Add a Media & Text block
4. Add an image to the media side
5. In the text content area, add a Paragraph or Heading block
6. Enable "Fit text" under Typography settings for that block
7. Observe: the text does not scale to fit the content area properly

Now open the same page in Chrome — the text scales correctly.

Expected behavior

The text should scale to fit the available width of the .wp-block-media-text__content area, just like it does in Chrome/Edge/Safari.

Actual behavior

In Firefox, the text either overflows the content column or renders at an incorrect (too large) size. Problem occurs on both the frontend and within the block editor.

Environment info

• Word