Hi! I'm afraid Mozilla doesn't allow separate beta versions hosted on AMO.
To install the new version with Vue3 support, go to https://github.com/vuejs/devtools/releases and expand the 'Assets' section, then click on the .xpi file. Firefox should prompt you to install the devtools. Please also disable the version you currently have (you want to have only one version active at a time).

Hi! To install the new version with Vue3 support, go to https://github.com/vuejs/devtools/releases and expand the 'Assets' section, then click on the .xpi file. Firefox should prompt you to install the devtools. Please also disable the version you currently have (you want to have only one version active at a time).

5.x doesn't support Vue 2. Please install 6.x beta from GitHub releases.
https://github.com/vuejs/devtools/releases/tag/v6.0.0-beta.15