bug(menu): opening CDK context menu loose text selection on Firefox

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

Hi there,

I’ve been working with CdkContextMenuTrigger in conjunction with CdkMenu and attempting to implement a copy/paste feature. However, I’ve encountered an issue specifically in Firefox: when the context menu opens, the selected text is unexpectedly unselected.

After some investigation, it appears that Firefox may automatically deselect text when focus shifts to elements outside the text container. While this behavior might be a limitation inherent to Firefox, I believe it is worth documenting.

Additionally, it is worth noting that the official CDK context menu example includes mocked "copy" and "cut" options, which exacerbates this issue.

Is this behavior something that can be addressed, or is it an inherent limitation of how Firefox handles text selection and focus?

Thanks!

### Reproduction

StackBlitz link: cf. https://material.angular.io/cdk/menu/examples
Steps to reproduce:
1. Go to https://material.angular.io/cdk/menu/examples with Firefox
2. In "Context menu" example, try to select a portion of text and right click on it
3. See the menu is opened but the text is no more selected

### Expected Behavior

The text should remains selected

### Actual Behavior

The text is no more selected

### Environment

- Angular: 18.2.3
- CDK/Material: 18.2.3
- Browser(s): Firefox 130.0 (64 bits)
- Operating System (e.g. Windows, macOS, Ubuntu): Ubuntu 24.04

Start by reproducing the issue in the official CDK context menu example with Firefox, then inspect CdkContextMenuTrigger and CdkMenu behavior around text selection and focus. Done means determining whether the selected text can remain selected when the context menu opens, or clearly documenting the Firefox limitation if it cannot.

Written by the indexing model from the issue text.