Broken sockjs connection when using proxy with firefox > 65

[NateScarlet]

Version

3.11.0

Reproduction link

https://github.com/NateScarlet/issue-repro/tree/vue-cli-1

Environment info

Environment Info:                                            
                                                             
  System:                                                    
    OS: Windows 7                                            
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz    
  Binaries:                                                  
    Node: 10.16.0 - C:\Program Files\nodejs\node.EXE         
    Yarn: 1.16.0 - C:\Program Files (x86)\Yarn\bin\yarn.CMD  
    npm: 6.11.1 - C:\Program Files\nodejs\npm.CMD            
  npmPackages:                                               
    @vue/cli-overlay:  3.11.0                                
    @vue/cli-service: ^3.11.0 => 3.11.0                      
    @vue/cli-shared-utils:  3.11.0                           
    @vue/component-compiler-utils:  3.0.0                    
    @vue/preload-webpack-plugin:  1.1.1                      
    @vue/web-component-wrapper:  1.2.0                       
    vue: ^2.6.10 => 2.6.10                                   
    vue-hot-reload-api:  2.3.3                               
    vue-loader:  15.7.1                                      
    vue-style-loader:  4.1.2                                 
    vue-template-compiler: ^2.6.10 => 2.6.10                 
    vue-template-es2015-compiler:  1.9.1                     
  npmGlobalPackages:                                         
    @vue/cli: Not Found                                      
                                                             

Steps to reproduce

1. git clone -b vue-cli-1 https://github.com/NateScarlet/issue-repro.git
2. cd issue-repro
3. npm install
4. npm run serve
5. Open http://localhost:8080/abc in Firefox 68

What is expected?

Sockjs connection established correctly

What is actually happening?

Can not establish connection.

---

Related to:

https://support.mozilla.org/bm/questions/1253577

https://bugzilla.mozilla.org/show_bug.cgi?id=1536843

facebook/create-react-app#6720