From version 19.12 of Citrix Virtual Apps and Desktop, it has been introduced the receiver detection (on the storefront component) for Linux O.S.
On the client side, the receiver detection is based on a specific Citrix plugin (type: npapi). These types of plugins have been deprecated in most of the browsers (Firefox, Chrome, Safari) except in Internet Explorer. Particularly, from Firefox v52 the support to npapi has been removed and the automatic detection of the receiver does not work.
While on a standard PC, the receiver is detected the first time and then the cookie is not asked again; on a thin client, cookies are removed at every startup. The best solution to this problem is removing the control on the storefront side by modifying the web.config (e.g. C:\inetpub\wwwroot\Citrix\StoreWeb) inside the directory of the store (as indicated on https://support.citrix.com/article/CTX202461) or removing the control on the Linux systems by deleting the word Linux of that file: modifying:
<protocolHandler enabled="true" platforms="(Macintosh|Windows NT|Linux|CrOS).*((Firefox/((5[2-9]|[6789][0-9])|\d\d\d))|(Chrome/((4[2-9]|[56789][0-9])|\d\d\d)))|Macintosh.*Version/(1[2-9]|[2-9][0-9]).*Safari/" skipDoubleHopCheckWhenDisabled="false" />
with
<protocolHandler enabled="true" platforms="(Macintosh|Windows NT|CrOS).*((Firefox/((5[2-9]|[6789][0-9])|\d\d\d))|(Chrome/((4[2-9]|[56789][0-9])|\d\d\d)))|Macintosh.*Version/(1[2-9]|[2-9][0-9]).*Safari/" skipDoubleHopCheckWhenDisabled="false"
Read more on: How to Disable Receiver Detection