You have SharePoint 2013 On Premises Deployment. When you try to open the files from your SharePoint sites, with file extension .MHT or .MHTML, the files get opened via Microsoft Word.
You want these files to be opened by your Browser instead.
The file open behavior is driven by DOCICON.xml file under the SharePoint configuration file. This file is located under - C:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15TEMPLATEXML folder.
In this case, MS Word in configured as the file open application DOCICON.xml file like below.
You will need to update this setting like below in the DOCICON.XML on all the SharePoint Application and Web Front End server of the Farm. Perform the following changes on each server such that the Edittext/OpenControl/OpenApp is not set in which case the browser end user client File Association settings will take precedence. Where by default the file extension association is Browse for .MHT and .MHTML on the user client desktops.
After saving the above settings, restart your IIS server to re-cache the new settings to take effect.