This is limited because setTabProxy checks if tab.url starts with https:// or http://. New blank tab has about:blank URL (at least in Firefox).

This also makes it impossible to set tab proxy for a page that failed to load in new tab because it's only available through proxy, one of these scenarios:

• Open new tab
• Enter URL of some page that is only available through proxy (i.e. inside internal network or somehow firewalled for other reasons)
• Press Return in URL bar
• Page fails to load
• Press FoxyProxy toolbar button, select proxy, press "Set tab proxy" button
• Try to load page again, page still does not load, proxy is not set

or:

• Click a link (to a page that is available only through proxy) in existing page with middle mouse button or with control key to open it in new tab.
• Page in new tab fails to load (it can be only opened through proxy)
• Press FoxyProxy toolbar button, select proxy, press "Set tab proxy" button
• Try to load page again, page still does not load, proxy is not set

In both cases, until page is loaded to some extent, tab.url attribute is still about:blank, so "set tab proxy" option does not work.

I tried to bypass that check in lines 225-229, it works, at least in Firefox and fixes described scenarios. Maybe it's safe to remove such check (remove whole switch statement) or to allow about:blank.

Try this and see how it works

switch (true) { 
  case !/^(https?:\/\/.+|about:newtab)$/.test(tab.url): // unacceptable URLs 
  case this.bypass(tab.url):                            // check local & global passthrough 
    return; 
} 

When I enable proxy with some url opened and then press home button and it opens new tab in current tab, proxy is still enabled and I can open another url with proxy already enabled.

Tried, it works, but in latest release (v8.9 tag). It does not work on master, where Proxy.onMessage for some reason is not called at all in new tab when selecting proxy in dropdown.

On v8.9 it works, but scenario with opening a link in new tab that I described above does not work because URL is about:blank in that case and about:newtab in "open new tab then enter URL" scenario.

So, this change worked for me:

diff --git a/src/content/on-request.js b/src/content/on-request.js
index 2c5922f..a131db0 100644
--- a/src/content/on-request.js
+++ b/src/content/on-request.js
@@ -228,7 +228,7 @@ export class OnRequest {
   static async setTabProxy(pxy) {
     const [tab] = await browser.tabs.query({currentWindow: true, active: true});
     switch (true) {
-      case !/https?:\/\/.+/.test(tab.url):                  // unacceptable URLs
+      case !/^(https?:\/\/.+|about:newtab|about:blank)$/.test(tab.url): // unacceptable URLs
       case this.bypass(tab.url):                            // check local & global passthrough
         return;
     }

Also tried it in Chrome, but seems that "set tab proxy" is not possible / not implemented in Chrome. I don't have Android devices to test Android Firefox extension.

Found out that in main, popup code has its own check for acceptable tab url. After these changes it works for me on main:

diff --git a/src/content/on-request.js b/src/content/on-request.js
index d327f02..1f4853a 100644
--- a/src/content/on-request.js
+++ b/src/content/on-request.js
@@ -223,8 +223,8 @@ export class OnRequest {
   static setTabProxy(tab, pxy) {
     // const [tab] = await browser.tabs.query({currentWindow: true, active: true});
     switch (true) {
-      case !/https?:\/\/.+/.test(tab.url):                  // unacceptable URLs
-      case this.bypass(tab.url):                            // check local & global passthrough
+      case !/https?:\/\/.+|about:blank|about:newtab/.test(tab.url): // unacceptable URLs
+      case this.bypass(tab.url):                                    // check local & global passthrough
         return;
     }
 
diff --git a/src/content/popup.js b/src/content/popup.js
index 893e851..0ea0fbc 100644
--- a/src/content/popup.js
+++ b/src/content/popup.js
@@ -104,7 +104,7 @@ class Popup {
 
   static async checkTabProxy() {
     const [tab] = await browser.tabs.query({currentWindow: true, active: true});
-    if (!/https?:\/\/.+/.test(tab.url)) { return; }         // unacceptable URLs
+    if (!/https?:\/\/.+|about:blank|about:newtab/.test(tab.url)) { return; } // unacceptable URLs
 
     this.tab = tab;                                         // cache tab
     const item = await browser.runtime.sendMessage({id: 'getTabProxy'});

It is done for v8.10