This addon enables opening/viewing .vsd, .vdx, and .vsdx Microsoft Visio file formats in Firefox.

Judging by its comments, each document must first be uploaded/saved to the developer's cloud server prior to viewing. If you work for a company or have a sense of privacy, then this will be an issue.

While the addon may be indicated as having a BSD licence, its source appears to be a link to a software as a service. In this case, you might be better off with something like a locally hosted, open source Node or Python server.

There are two ways to check a file. The first is to download it. This way you can look at it all you want until you remove it.

The second is to ‘view’ the file. In this case, the file is still downloaded but to temporary storage. Note that sooner or later, all temp files will be removed by the system.

You can, however, save the temp file via the Save option.