docs: link to Firefox extension

[skirtles-code]

Fixes #1073.

I've added a link to the Firefox Extension in both README.md and /getting-started/installation. Rather than linking directly to the extension download, I've linked to /guide/browser-extension, which already has the download link.

I've also updated the links to the Chrome Extension to link to /guide/browser-extension, rather than linking directly to the Chrome Web Store. This helps to keep all the information about installing the Chrome Extension in one place.

I reordered the links in README.md to match the order in /getting-started/installation.

In UseModeList.vue I also removed some code from the template for handling disabled and blank links, which is no longer required and causes type-checking errors.

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commit: 39f45af